The main reason is that when the baby is sucking on the nipple, it will stimulate the uterus to contract, so there will be a painful sensation of menstruation. If you have been breastfeeding for more than two months and still have the painful feeling when you are about to have your period, it is clinically possible that the pain is caused by inflammation, so it is best to go to the hospital for an examination. Although the vast majority of women have menopause during breastfeeding, some people will resume ovulation. If you have sex during breastfeeding, you must take birth control measures, and if you are pregnant, it may also lead to pain in the belly.