What Causes Pink Discoloration Of Penis Tip?

hii sir , I am 22 years old ,my penis tip is turned into pink color some portion is remain as black only . its increasing slowly . I am not feling any itch or pain over there . at it started as a small dot but now it spread all over the tip . I feel embarrassing to approach doctor .could you please suggest some medicines so that i can be cured . please help me