Food allergies ???

My baby turned 5 months old last week and my doctor gave me the ok to go ahead and try him on some baby cereal. Told me to just make it really runny .. I tried him on it for the first time yesterday morning around 9 am.. by around 1 pm there were a couple little spots on his chin. We didn’t think anything of it and went about our day. As the day progressed he developed and pretty bad rash all over his face and body. Last. Ugh I decided to take him to the emergency room because it had gotten pretty bad. The doctor told me she really didn’t think it was an allergic reaction because he would have other symptoms like swelling around the lips/ mouth, vomiting, diarrhea… and so on. He did have some hives but she told me the most common reason for rashes/ hives in babies and young children is actually viruses. She gave him a dose of reactine there and told me to use that at home today as well if rash is still present. She said other symptoms will likely occur in 24-48 hours and said to go ahead and give him

More cereal in the morning. I woke up this morning and his rash was mostly gone. Very very little was left. I gave him cereal and within a few hours the same exact thing started to happen.. as the day went on his rash got prettty severe. I gave him some reactine tonight ( had to wait 24 hours after first dose to give him more) within 30 mins his rash slowly started to fade. It’s still there now 1.5 hours later but very mild and not as bad. I really feel like it’s likely a reaction to his oatmeal. I’ll include some photos . Have anyone had a similar situation.

* I am going to stop giving him the cereal and make an appointment with my family doctor to discuss this. But may take some time to get in.