Suggest Treatment For Irregular Periods

hi, i really want to know whats wrong with me... my period has always been messed up when it wants to be, but i've never had it for 18 days strait and its still not over. i've had sex for the first time a while ago and i feel like that could be a reason why. i've been told it was messed up because of stress before and then it eventually got back on track, but i've never had it this long??? i want it to STOP!! i just don't know what to do! Please Help..