Rash?

Currently just over 5 months pregnant, with my first. Never had anything like this before. My boobs haven't changed in size and neither has my width around my breast area (got measured for new bras). Thinking it may be the underwire? Maybe the heat (it's been awfully hot here for this time of year). Wondered if anyone had suggestions to help it go away? It's a bit itchy and the skin is slight try/flakey. Sorry for the photo.