Painful bumpy rash under my breast. only when i wear underwire bra. allergy?

Underwire Bra: You may have an allergy to the underwire of your bra especially if the wire has nickel. The other possibility is that the underwire is too irritating to your skin.

Contact allergy?: U could have developed contact allergy to the metal in the underwire. U should see a dermatologist or an allergist for a patch testing. Contact allergy rash can be burring and red. Sometimes in women with larger size breasts, there can also be a yeast infection, called candida, developing under the breasts. It can be bright red and painful sometimes. Need to see a doctor.

Possibilities: I agree w the 2 other answers you received. 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.