What Causes Rash On Buttocks With History Of Bunion Surgery?

I have a rash started on my buttocks 3 days ago. I now have it, although much slighter, on my shoulders. Bumps are fairly spread out, not close. They only itch if I scratch them and then not much and the itch is easily ignored. I can think of nothing new I am doing. I had bunion surgery a month ago, at which time I took percocet for pain for a week an a half, but have taken none for about 20 days. I also recently increased the amount of OTC extra strength acetominophin from one 650mg tablet to 2 for degenerative arthritis in my shoulder.