What Does Bleeding Between Periods Indicate?

im having a period every two to three weeks. what could be the reason? I was a late bloomer and ddint start untill i was 15.I have never had a normal period.I once went 7 months without one. I went one birth control to regulate them but got off of them because i was haveing headaches. Its been almost a year in a half now and i have a period alomost every month.