pain when running

I am 17 and run every night for 20 minutes. I have for 2 years, but earlier this spring I experience severe cramps in the uterus. I had to stop, even then it got worse until I was on the ground. I bled a little after. I waited a few days and could run again fine. It happened rarely, but a few weeks ago it happened so bad after such a short time and was the worst pain i'd ever felt. I have no problem with pain, but just remembering that pain is scary. Any idea what could cause it? I'd really like to run again.