Hi Doctor I Have A Brown Discolouration Between My Breasts...

Hi Doctor

I have a brown discolouration between my breasts... It doesn't hurt nor does it itch I'm worried that I have breasts cancer ... It gets lighter n then darker sometimes. What is this ?? I went to the doctor and he said I should use a cream called viocort. . He said I should use this for 2weeks and also said that it is common .. However it did not go away and it is still there.
I have white heads and some black spots that protruding from my breasts .. please help it's honestly a worry for me everyday and would really need some advice as to what I'm doing wrong and what could help me :-(