My period comes on every 3 weeks for 3 days. I get severe cramps, headaches, and diarrhea.

Ob-Gyn can evaluate. Menstrual cycles are usually 28 days, but a few women have normal 21-day cycles. However, severe cramps are not normal, so an ob-gyn doctor can evaluate to see what the problem is, and to see why there is also diarrhea and headaches. Severe menstrual pain may be endometriosis, which causes some bleeding inside the abdomen during a woman's periods, and needs medical treatment.