My son gets red cheeks also, I'm suspecting food allergy/sensitivity. I've heard some people say it's due to teething, but that only makes sense if they are drooling a lot and the moisture is irritating the skin. I can understand the drooling causing a rash around the mouth, but not the cheeks. I just read Is This Your Child by Doris Rapp, MD. and she shows a picture of a child with red cheeks and it looked just like my son. Like big red circles on the cheeks.
I've noticed him get it right after eating something and he almost always gets the red cheeks after nursing (sometimes only on one side).
Good luck. This food allergy stuff sure is frustrating