When I first got my period at 11 I had a predictable, 7 day, cycle. It wasn't the perfect 28 days but I knew about when it was coming.
At age 15 I started missing my period. At first it was every now and then, then it became more often. At times there would even be 3 months between each period. They also became lighter only lasting 3 days. When I looked it up online a condition called oligomenorrhea seemed to match it.
I was 19 by the I first went to an ob/gyn. They just pushed be aside and put me on birth control to make me have a 28 day cycle. I was on it for a year, the month I got off it my period was at 35 days instead of the 28 it was at for the entire year before. I don't think it worked in the long term for me but I am unsure how long you need to be on it for it to correct something like that.
A month later I went on the depo shot so I no longer get my period.
I am 21 now and I know no idea way my period suddenly became irregular. People always told me that i'm young and it usually takes till your 16 to start a regular cycle, I seemed to go the opposite direction. The only reason I can think of it that I lost my mother when I was 11. I know when females live together they all have their period at the same time due to the chemicals we give off. Well I only lived with my father so I do not think my body had another female to put in on the right cycle. Just a thought.