What you are saying sounds very distressing, and my older son has somewhat of a similar problem.
This is what I would do. Try to find cotton bras, like the type of sports bras. Sometimes, when you tie the back straps, it helps take even more weight of breasts.
Make sure your skin is very, very dry after you shower and then apply under your breasts a 'zinc oxide powder' which will help keep the area dry and the zink oxide is so healing. You could even get a tube of xink oxide and apply the cream at night in that area under your breasts. (it's a little messy)
Then, with my son and also his father the major thing is nuts and chocolate, he breaks out immediately.
Try that and if this is not sufficient you may want to try an anti-fungal cream like Lotrimen sold over the counter.
Good luck, Kat
You ideas as to what these may be sounds very likely. I would try to keep the area as clean, dry and irritation free as possible and you might consider seeing a Dermatologist as this may be just some type of skin problem. Regards .....