What Causes Darkening Of Skin On Stomach,thighs And Knees?

Hello doctor, I really need your help at this point of time. I had a fair skin but suddenly i figured out that the skin colour of my stomach,butt,thighs,knees,legs and even my shoulder started to blacken very badly. I actually feel disgusted looking at my self now specially my stomach ..it has turned really black. I mean like how is that possible? It is least exposed to the sun.my chest colour also seems to become dull and the fairer skin fade away.this has really started to scare me now. I am really tensed and bother. Please kindly help me with the best suggestions to gain back my fair complexion. Lastly is it really possible to gain it back?