My 1 month year old started getting a bad case of what appears to be dermatitis (baby acne combined with cradle crap). I called my ped and he was saying many babies develop around this time and not too worry too much.

I 'm sure this is the case, but I also read that skin rashes can develop as a result of an allergic reaction to formula. I usually supplement 1-3 feedings. I've been using the same formula he was fed at the hospital (Similac Advanced Shield). As far as I can tell, my LO does not seem to have any problems with it other than some spit up (not a ton) and some extra gas.

I guess my question is if he was allergic to the formula wouldn't he have had some type of reaction earlier than now?

Just trying to eliminate all the possibilities for his rash.