What Causes Dark Skin Around Vaginal And Anal Area?

I am 18 yrs old and as far as i can remember I have had really dark skin in my vaginal and anal area. IT is close to black. I do not know what to do about it, It is very embarrassing and makes me feel uncomfortable about myself. I have the same problem with my knees, elbows and armpits. Is this something normal? should i be worried?