i'm in a similar situation, except i have never had sex or a pregnancy risk.
my period had been regular for almost 10 years up until february. it didn't come at all. i was highly stressed out that month though so i decided to wait until the next month. it came in march and i figured missing once was no big thing.
but then i skipped april and may and couldn't figure out why because i wasn't that stressed during that time.
my doctor ran a bunch of tests on me last month. i have a family history of diabetes (grandfather) and also hypothyroidism (mom). turns out neither were the problem.
i went back today for a pap smear and my doctor put me on birth control. so everything should start to regulate soon. she is going to do an ultrasound next month to check for ovarian cysts.
so if you have a family history of endocrine problems or anything, it is worth asking your doc is those could possibly be factors into why you are having an irregular period.
also, as everyone here seems to affirm: sometimes, for no apparent reason, your body can just get out of whack.
i'm still really wondering what is going on but i may never know. all i can do is let my doctor treat me and get on with my life.
like my mom said to me today, hormones are crazy things and they can get messed really easily.
so just take care of yourself as usual, do what you can to be healthy, and i'm sure you will be fine and everything will straighten out soon.