When you have your menstrual period, you may get mild to bad menstrual cramps or pain in your abdomen or back. You may also have nausea, vomiting, diarrhea, constipation, a headache, or lightheadedness. Some or all of these problems may start a day or two before your menstrual period and last for part of or all of your menstrual period. Menstrual cramps are caused by contractions of your uterus as the blood leaves your uterus. (Read more about Menstrual Cramps)