There's some information here about milk blisters. They are usually pretty straightforward to treat but there are lots of ideas there if home remedies don't work.
It is OK to wear underwired bras, but only if they have been fitted properly. A bra that's the wrong size (usually cup size too small and back size too big) can cause problems. You might want to have a chat with the MN bra gurus over on style & beauty, thread here.