Why Am I Having Menses From Past 25 Days?

i'm 20 years old and i'm having problem of menses since i was just 13 i have consulted many doctors but there is no result they have taken all my medical tests but they say there is nothing wrong inside according to the tests.now i'm having menses for atleast last 25 days and sometimes they are more and sometimes less this time i haven't consulted a doctor because it's useless..when they give medicine i become alright but when i stop taking medicine i have to face the same condition again...can you please help me.....