After pregnancy on day 40, I started my period. They were very heavy a…

After pregnancy on day 40, I started my period. They were very heavy and the gynecologist said that the body is already ready for another pregnancy and prescribed me birth control pills. I take them every day at the same time, never missing one. But here's why I don't have my next period ? it's been two times now. What kind of nonsense? What could this be?