Depo shot

Im 34 and decided to go back in birth control. I decided on the shot. This is the first time I'm on it. I went on June 27th and should have gotten my period July 4th. The doctor said that the shot would change my cycle and could eventually stop my period. However it is July 18th and I still haven't gotten my period. My doc had me take a pregnancy test 2 weeks after I got the shot because I had sex June 18th. The test was negative. Could taking the shot stop my period this soon?