Nipples become hard during ovulation, which is a very common phenomenon during the female menstrual cycle. During ovulation, a woman's breasts become firmer and her nipples become more sensitive due to changes in hormone levels.

For some people, stiff nipples during ovulation may cause discomfort or pain. If you're experiencing this, consider taking steps to relieve discomfort, such as applying massage oil or olive oil to your nipples, or gently massaging your breasts.

In addition to nipple hardness, women also experience other physiological changes during ovulation, such as increased body temperature, increased appetite, and increased energy. These changes help promote physical health and fertility.

While having hard nipples during ovulation may be uncomfortable, it's not a bad thing. During ovulation, a woman's body is preparing for upcoming sex, which can help increase sexual desire and intimacy.