Help! seriously worried about irregular periods!

Hi, im 17 years old and am still a virgin. i got my first period when i was 11 and as far as i can remember they we're regular. But around the ages of about 14 they started messing up and sometimes i would get pain but not very often, but now over the past 2 and a half years my periods are very irregular and the symptoms over the past year seem to be worsening in some ways. I have a period every other week, sometimes 13 days or more; i stay on my period for about 7 days or longer, sometimes 14!! and sometimes, not all the time i get really bad pain in my lower belly that makes me want to be sick. i also get low back pain and always have blood clots.The last time i was on my period i was weeing alot, so i went to the doctors concerned it may be a cyst and he pressed on my stomach and said he couldnt feel anything so probably wasn't anything there, he also said that because i was stressed 2 years ago because my parents divorced that could have caused it...but that was 2 years ago why even when i am not stressed do i have irregular periods? Could it be endometrosis or would i have to be in pain every time on my period?? please help x