What Are The Irregular Heavy Periods With Constant Cramps?

Hi i'm 17 and had a baby almost a year ago (extremely premature but natural delivery), after I had 3 regular periods and then they stopped. I get constant cramps and I think i've had 2 periods since, but they were very heavy and clotted.. seen a gynae but he said all was fine. I'm worried as I get permenant pain..