During menstruration your body makes chemicals called “prostaglandins.” Prostaglandins cause the uterus to contract or tighten in general. It’s the same kind of contraction that happens during labor or childbirth. Such contractions during a period are normal. However, they can be painful. The medical name for painful periods is “dysmenorrhea.”
Medical conditions can make the pain during your period worse and need to be accessed by a qualified doctor. The most common condition to be investigated is called “endometriosis.” In this condition, tissues that should grow only in the uterus grow outside the uterus.
Women with painful periods have cramping in the lower belly. The cramps can be mild or bad. You might also have pain in your back or thighs. Pain often starts with your period or right before your period.
Some women also have:
●Bloating (a feeling of fullness in the belly)
Source: (53) Puneet Chandna’s answer to Why is there so much pain during a menstruation period? – Quora