Suggest Treatment For Irregular Menstruation

I have been pretty irregular on my periods, since the first one I had when I was 9 yrs old. One time my mom's doctor prescribed birthcontrol pills for me and after i stoped taking them my period went irregular again so I just got used to it. Even though they were irregular they all lasted about 5 to 7 days and the flow and cramps were not that strong, pretty normal I guess. The weird thing I have recently noticed is that the last three periods Ive had have just lasted 2 or 3 days and they are pretty much only BIG blood clots, there is low if none "flow", and the cramps are horrible. I am 18 now and have never been to a gynecologist. Im just really concerned about it because it doesnt seem to be normal to me...