Why Am I Missing My Menses After And Implanon Implant?

Hi, I got the implant 2 years ago in April, I dont get a periods which I know this happens but I have gained around 4 stone since then, I am now 11 and half stone at 5 ft 3 at the age on 20. I have got stretch marks on my legs and hips, my belly looks pregnant, my appitite has got bigger since I got it and I crave fatty foods, ive tried dieting etc but it just doesnt seem to budge. Help?