Why did my lower stomach hurt so much?

I woke up in pain, in the front of my lower stomach. My legs ached too and it just kept getting worse until I was writhing in pain and vomiting, I almost ped out. My period started today, but it started after the pain finally ped (after about 45 minutes). The pain came in waves and was very intense. What could it be? (it was around where my uterus is)