It is normal to have stomach pain on the first day of menstruation. This is because during menstruation, the endometrium causes a small amount of shedding, which causes contractions of the uterus when it is shed. As the contraction needs to be rapid on the first day, it can help the endometrium to shed slowly, and it takes a process to change from a thicker endometrium to a thinner endometrium, which may be more painful at this time. And when the uterus is contracting, it may also cause confusion in the intestinal function, some people may experience nausea, vomiting, or even diarrhea, which is in the menstrual syndrome. You can drink a lot of hot boiled water or brown sugar water, and if necessary, you can drink some motherwort granules can help relieve. If the pain is particularly unbearable, it is recommended to go to the pharmacy to buy ibuprofen extended-release tablets or capsules, which can be taken orally and will not affect your health.