I just read the symptoms of inflammatory breast cancer on the web sites of the Mayo Clinic and the American Cancer Society. A blister on the breast was not listed as a symptom, and of the symptoms which were listed, you have not mentioned a single one.
Even if it had been listed, wouldn't it be a remarkable coincidence that the blister appeared in the exact same spot as a blister that had been caused by your bra would have appeared?
Of course, I am a Grandma, not a doctor, and I might have missed pertinent information. Please do not take my post as indicating that you do or do not have cancer. I'm just sharing information which I think is correct.