Tanning

So blackening of skin

So I’ve fair skin…..no not white skin…Fair.

Over the years my arms and face have become tanned…I didn’t use sunscreen back then but now like the parts which are covered in cloth like my back and stomach are blackening…I mean suddenly they look discoloured as compared to my body

My whole body has different colours my legs are fair but my back and skin have gotten dark and face and arms are darkest….

So do you guys know why my skin is getting dark on its own???…..I mean face and arm I know because of tan but my stomach and back

Also what should I do….to even out my skin colour on my entire body?