Breakthrough Bleeding, Missed Periods...?

Over the past 6 months, my period has been completely out of whack, sometimes late for weeks or missed all together for months at a time. Over the past two weeks, i've started having breakthrough bleeding on a regular basis. And my neck and joints are in constant pain. I'm not on the pill, never have been, and NOT PREGNANT. I know this for a fact, what else could it be?