What's going on with me?

When I was 12, I was put on birth control pills to make my period regular. They would be on for weeks! Heavy with blood clots. Later on at age 15, I was taken off the pill because my period became normal. Only on for 3 to 4 days and light or normal.I was really supose to be taken off the pill at tge age of 16. I turned 16 sunday (april 4). And my period is back! It's heavy and with big blood clots that kept coming. It was so bad that I called my mom to get my because I thought that I wasn't going to make it the whole day...I have no idea what's going on with me.What is causing me to have this crazy problem. I have never had in my life or anything like that! Can anyone help me Please!?