No period at all while on the depo provera shot for the last year

Patient: I had a baby Sept 2010 and opted to use the depo provera injection at 5 wks post partum. I have not had a period during the year I’ve been getting the injections, technically i have not had a period since dec 09(when i became pregnant with my son). Is this normal? my dr said it was “not unusual”, but I’m a bit concerned.