What Causes Irregular Periods?

My period comes on whenever it want to. Sometimes it heavy and other times its so lite that all i need is pantie liner. I also been having like 14 day long cycles. For about the four mouth my cycle was every month on the 18th then the dates changed and moth of May my period started on the 24 and went off about 6/4. since my period went off I have been sexually active and yesterday I woke up with bright orange blood. then it was gone what could it be. I dont have kids and never pregnant.