What Causes Missed Periods After Taking Injections For Birth Control?

I have been on the injection for over a year and my monthly peroid went aways after the first month of the 2 month injection. Went to the doctor a few months ago and advised me to go off for a while because my system rejected the injection. I am now off for more the 3 months and I picked up weight, my face is just spotted out and my system is off, I feel dissy and eating alot. Dont know what to do. Please help