What Causes Scanty Menstrual Period While On Slim Fast Diet For Weight Loss?

hi, i started the slim fast diet about two months ago andd i am losing weight everywhere apart from my belly. my belly is gainin andd av put half a stone more on since starting. i have come on my period vut its not a proper one andd am losing my appetite. whats wrong with me ? i am scared how quickly i have put weight on.