Menstruation, or period, is normal vaginal bleeding that happens as part of a woman's monthly cycle. Many women have painful periods, also called dysmenorrhea. The pain is most often menstrual cramps, which are a throbbing, cramping pain in your lower abdomen. You may also have other symptoms, such as lower back pain, nausea, diarrhea, and headaches. Period pain is not the same as premenstrual syndrome (PMS). PMS causes many different symptoms, including weight gain, bloating, irritability, and fatigue. PMS often starts one to two weeks before your period starts.