Red blotches

My son is 8months old gets red blotches around his mouth when drinking. He has been on breastmilk since birth. I am going back to work next week and have since introduced Similac Formula. Now he is getting red blotches around his mouth when he drinks. But not anywhere else, just around his mouth and chin. It goes away very quickly and doesn't seem to bother him.

I was worried it was the silicone on the nipple, but water and juice give no blotches. I called my pediatrician and the nurse on call couldn't figure it out. She said try a different formula, but I don't have a lot of time. Has anyone experienced that same thing? And what do you suggest I do?