What Causes Delayed Periods After Removal Of Nuvaring?

I was on the Nuvaring as my method of birth control. The other day I took it out and a day later went to the gyno to ask about switching to Depo-Prevara instead. She determined that was fine and gave me the shot. It has been a week since I took out the ring and I still have not gotten my period. Is this normal? I am not pregnant.