Depo Provera

Patient: Hello, I started taking depo provera in aug 2010 at my 6 week check up after having my son. Well i was due to get my last injection oct 272011 but i decided to get off of it and get put on the pill. My dr put me on ORTHO TRYCYCLEN without my periods being regulated first. I started my 2nd pack of pills today and i still have NOT had a period. i have missed 2 pills and had unprotected sex. Im just curious as to why i have not started my period even though i am off the depo? thanks