It's probably heat rash from the sweating. Try using baby powder under your breast to keep them dry.
I had something similar very recently except it was on the top of my breast and extremely itchy and lasted over a week! It almost seemed like an abrasion but it couldn't have come from My bra or clothes, it was very strange. A few times the itching was so severe I used a little Benadryl cream, which I read is okay to use. It's now totally gone, I guess,just one of those things. I have,pretty sensitive skin and am only 12 weeks, so I guess I should be prepared for it to happen again!