Another symptom of PMS that is often mistaken for the actual period is:
This is incredibly painful and, one description I've heard is that it feels like lightning striking, a thousand knives being plunged into, and a sledgehammer crushing the pelvic area all at once. The intensity of the pain varies from female to female. Several explanations have been given for cramping, including poor circulation in the pelvis and the muscles of the uterus contracting to aid the flow of the blood, but obviously no one knows for sure.
It hurts like hell, though.