What Could Cause Missed Periods And Negative HPT After Weaning Off Depo Shots?

I'm 23 years old and for the past 2 years I was on the depo birth control shot. I decided I'd like to go back on the pill so I decided to go off the shot. I had my last shot in September and have not started my period yet. I've taken numerous pregnancy test and all are coming up negative. Is this normal?