I have sensitive skin and DD size boobs. Underneath is also very sensitive. I have broken out in rashes under my boobs many of times. Last night during sex my dh grabbed one and kind of did this lifting up thing on it. It hurt right away and I made him let go. I noticed during my shower this morning that even putting soap there made it sting. It's raw again. (just under that one though) he told me that because of this he thinks the solutions would be a reduction. My thoughts on this, is even if I got down to an A cup it still wouldn't help.
Weight loss is going to be your best bet. I have a similar problem, it was fixed with losing some weight and getting bras that fit properly. You can try putting some gold bond powder under them to keep the sweat at bay.
my great aunt used to get yeast infections under her boobs (I come from a large titty family) lol she ended up getting a reduction.. she was like HHH or something. You could be getting yeast infections.