It can happen out of nowhere. There you are, standing in the checkout line at a grocery store, when all of a sudden your nipples become erect. You don’t know why it happened, and you may be a little worried. Don’t be! Random nipple hardness is totally normal from time to time.

The nerves in the nipple react to stimuli, both physical and psychological. So an arousing thought, change in temperature, or something as simple as the fabric of your shirt brushing against your skin can cause one or both of your nipples to become erect.

However, there are some underlying health issues that can also lead to nipple hardness. Learning the signs can help you recognize when it’s time to see your doctor.

Sometimes, the products we use on our breasts can make our nipples hard. It’s most likely because you have an allergy or sensitivity. Soaps, shower gels, and lotions can trigger an allergic reaction. So can certain laundry detergents and fabrics.

Nipple hardness is just one sign of an allergy or sensitivity. Other symptoms you should watch for include:

  • redness
  • itching
  • chapping
  • rash

Truth is, ovulation is different for every woman. Not everyone will experience common symptoms that could tip you off that you’re ovulating. Breast tenderness is one of those signs, and it can lead to your nipples becoming hard. This happens because of a surge in estrogen levels.

Other ways you can tell you’re ovulating include:

  • changes in your cervical fluid
  • changes in your cervix’s position or firmness
  • a slight dip in your body temperature when at rest
  • light spotting
  • pelvic pain or cramping
  • bloating
  • increased libido

Breast changes and pregnancy go hand in hand. Fluctuating hormones and an uptick in blood supply can cause your breasts to go haywire, to be honest. Your nipples will stick out more and grow larger.

You may also experience:

  • your breasts and areolas becoming bigger
  • your areolas becoming darker
  • your breasts feeling tender and sensitive
  • thick, yellowish liquid known as colostrum leaking from your breasts

There are so many changes your body goes through during perimenopause and menopause, it’s hard to keep up. Breast tenderness is a common sign of perimenopause.

This is because your estrogen levels decline as you get closer to menopause. It’s not common, but it’s possible that your nipples become hard because of the changes in your breasts.

You can also expect the following during this stage of life:

  • irregular periods
  • problems sleeping
  • hot flashes
  • mood changes
  • vaginal dryness
  • trouble peeing
  • changes in sex drive
  • decreased fertility

Postmenstrual syndrome is just like premenstrual syndrome (PMS), but on the other end of menstruation. Many of the symptoms are the same, including breast swelling and tenderness. And for some women, that could also mean their nipples become hard once in a while.

During postmenstrual syndrome you may experience the same changes to your mood, behavior, and appetite, along with some of the same physical symptoms that you would with PMS.

This includes:

  • muscle pain
  • bloating
  • feeling tired
  • having a lack of energy
  • headaches
  • cramps
  • low sex drive
  • constipation or diarrhea
  • acne flare-ups
  • food cravings

Your nipples can become more or less sensitive after you have them pierced. This means that your nipples could become hard more often than they would’ve if you didn’t give it some bling. They may react quicker to stimuli like fabric or someone’s touch.

But while nipple piercings may look cool, they do come with some risks. Mainly, bacteria can enter your breast from the hole created by the piercing, even if you removed the jewelry and let the piercing heal. Bacteria that enters your breast can lead to mastitis, an infection of the breast tissue that causes nipple hardness.

Other symptoms of mastitis that you should keep an eye out for include:

  • breast pain
  • swelling
  • warmth
  • redness
  • fever
  • chills

Your baby may have a hard time finding your nipple when they’re flat or inverted. That’s why erect nipples are vital to breastfeeding — they help your baby. Your nipples may also become hard when your baby feeds because of the stimulation.

But hard nipples during breastfeeding could also be a sign of mastitis. In fact, breastfeeding is one of the most common causes of breast tissue infections in mothers. Mastitis usually happens to breastfeeding mothers in the early days of childbirth, either because of clogged milk ducts or bacteria that snuck into the breast through a chapped nipple.

Bacteria that enters the breast either through a cracked or pierced nipple can cause a buildup of pus, leading to a breast abscess. This is a very painful condition that can trigger hard nipples. Abscesses usually form in the breast when mastitis is left untreated.

Other signs of a breast abscess to look out for:

  • tender, red, hot, and painful breasts
  • fever
  • muscle pain
  • lack of energy
  • nausea
  • headache

You may know this already but nipples are one heck of an erogenous zone for a lot of women. That’s because the tingling sensation felt by your nipples travels to the same part of the brain that receives signals from your genitals. When you stimulate your nipples, your nerves tell your muscles in the area to contract, thus hardening your nipples. Your nipples can also become erect when you have a sexually arousing thought.

Of course, nipple hardness is just one symptom of arousal. You may be hot and bothered — even subconsciously! — if:

  • your heart is beating faster
  • you’re breathing faster
  • you become flushed
  • your vagina becomes wet or swollen

We’ve all been there: We’re bundled up, ready to brave the winter air, and bam, our nipples pop out. The rest of you may be warm, but that doesn’t mean your nipples can’t catch a chill.

In fact, cold weather is one of the most common causes of nipple erection. That’s because a drop in temperature simulates special nerve cells in our nipples — the same ones that cause goosebumps. Hot weather, though, doesn’t cause quite the same reaction with our nipples.

Let’s be honest: Visible nipples get a bad rep. That’s why the #freethenipple campaign started five years ago — in order to celebrate our beautiful nipples when they’re out and proud. But you should do whatever helps you feel comfortable in your skin, whether that’s covering up your nipples or letting them stand out.

If you want them to stay hidden, you have plenty of options. Padded bras, nipple covers, or Band-Aids can help keep nipples out of sight. You could also layer up or wear looser shirts if that feels better.

Shop for nipple covers.

If your nipples get hard at random, it can be totally normal. It can happen from time to time, completely out of the blue. And sometimes, there’s no explanation for it.

But if nipple hardness happens with other symptoms, like soreness or discharge, you should schedule a doctor’s visit to ensure there isn’t an underlying issue at play. And if your erect nipples are due to conditions like PMS or postmenstrual syndrome, menopause, or allergies, they’ll be able to help you treat the other symptoms you’re experiencing.

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