What Causes Irregular Periods With Heavy Clots?

I went off birth control December 2010, my last regular period (5-7 days) started Dec. 29th. I didn't have another period until Feb. 12th 2011 but it was really light and only lasted 3 days. Since then I have been spotting almost every day, every other day and twice I went a week without spotting.The first week of May I finally started a period, however that has now been a week and half ago and the blood flow has gotten progressively heavier and there have been a lot of blood clots. Yesterday I changed my regular sized tampon almost every 2 hours because I was bleeding through. Last night I woke up in the middle of the night and when I took out my tampon a blood clot the size of a tampon came out with it. I have not been experiencing any painful cramping like I normally would with a period. Probably on a scale of 1-10 only a 2 and the cramps only occasional. Should I be worried?