What Causes Stoppage In Menstruation?

I'm a twenty year old. I started menstruation at 12, had a healthy period for about 1.5yrs. My menstruation stopped thereafter. I have visted two gynecologists - both started me on a gestodene oral contraceptive to regulate my periods. However as soon as I stopped (after 1 year the first time, and two years the second time) my menstruation stops completely. I've been for blood test - no underactive thyroid, no sugar etc. Only high cholestrol (runs in family, taking a rosuvastatin containing medicine for that). I am overweight, weighing 73kg with a height of 1.65m. I lose weight on the contraceptive (dropped to 66kg). Why don't I menstruate?