What Causes Itching With Tiny Pustules On Back After Change Of Pain Medication?

I have been itching and getting small tiny pustules on my trunk, back of legs and arms. They appeared each time my dr. changed my pain medication prescription, but when I reported this to them, the nurse didnt think it was allergic response. It started about 8 hours after starting ocycontin. My tongue is also raw feeling....i am taking bendryl for the itching and using peroxide on the pustules. any clue what is going on?