Rash under my breast, is on both sides and only when I wear one bra with underwire. Could that be it?

Yes. It could be due to pressure, sweating or material itself. Acoid this bra.
Yes. Allergic reaction to metal in underwire is possible (especially nickle) however, direct contact with metal is not common. If the bra is worn out it could expose the underwire. However there is a thick cloth channel surrounding it & the wire is coated in plastic. The underwire can actually break. Yeast infections are not uncommon & excessive pressure from the wire against skin can be problematic.