I Have A Couple Discoloration Marks Under My Breast (purple

I have a couple discoloration marks under my breast (purple like in colour). When I touch them, I can feel a soft like bump (almost like a pimple). I'm 32 and my Dr looked at this before and suggested it could be from an ill fitting bra. I'm currently breast feeding and not going out much so not wearing a bra often so I don't think this is it. Could it be from sweating (I am a large breasted woman) or a block milk duct?