What Causes Weight Gain Despite Being On Birth Control Pills?

I was put on b.c. 6 months ago. Tubes tied almost 8 yrs now. Heavy bleeding & clotting. Thus reason for b.c. was bleeding 2 wks. Straight. Woke from dead sleep to need to change clothes. During day knew i needed to go to bathroom asap. On pill I am still bleeding. Not as heavy or clotting as bad but still with clots & length. Light pink bleeding for almost 1 whole pack of b.c. w/ some light pink tissue. Thyroid levels good & clotting levels good. Gained 20 lbs in last 6 months; even being on weight loss pills & working out. HELP!!!PLEASE!