What Causes Irregular And Light Periods?

My period has become very irregular over the past 6 months or so. It lasts about 8-12 days (used to last 5-6), only bleeds lightly for 1-2 days then heavy spotting the rest. It also comes on almost every 2 weeks, and one time I didn't have one for almost 6 weeks. I am on no form on birth control.