I have really bad periods (horrendous pain and sometimes sick - usually have to have time off work - when I was on the pill I just timed my periods so they'd be at weekends and I wouldnt have to call in sick).
I was diagnosed with endometriosis when I was 19 (now 26).
I'm not saying anyone here has got it (definitely hope you havent!), but very painful periods are obviously a symptom, as is losing old blood during period, and spotting between periods, and pain starting before your period starts. Oh and irregular periods can be a symptom.