I Noticed Skin Discoloration On Upper Belly And Under The

I noticed skin discoloration on upper belly and under the chest area about two weeks ago also under left axilla I thought it was due to friction of the bra so I try to limit to wear it but it didn't go away I don't have no pain and no itcing the only thing I feel very rare is slight burning sensation in my left armpit area I would like to know the reason and what should I do