What Causes Missing Periods While Off Depo Shot?

I was on the depo shot for about 2 years. I went off the shot. My next shot would have been January. It has been 3 months now, and I still havn't gotten my period. I did not get my period for the 2 years I was on the depo. I took pregnancy tests, and they all came back negative. why havn't I gotten my period.