milk allergy?

Ok, I'll try to keep this as short as possible...

DS is breastfed but we supplement 2 feedings a day with formula (Enfamil Gentlease). He has been on this since about 10 days old and seemed to do ok with it. He does have a lot of gas (don't all babies?!) and what I thought was baby acne on his face....

Well, today I had to go to a conference so my sister watched him all day. In a rushed attempt to get out of the house I forgot the pumped breastmilk. (Mother of the year right here! ) So I ran to the store and picked up some more formula for the day, meaning he only nursed this morning and had formula all day. When I called to check on him my sister said he was fine just extra fussy but she thought he just had a lot of gas and hadn't napped so he was tired. I picked him up tonight and he was completely miserable. When I went to give him a bath tonight I noticed he had a rash/hives all over his face and body. I immediately thought it has to be the formula and took to Google. Apparently it could be a milk allergy? He also just got diagnosed with silent reflux and a lot of the symptoms seem to be the same. He nursed tonight and is happily sleeping now but I'm freaking out a little. I feel so bad! Obviously, I'm calling his pediatrician first thing in the morning but was wondering if anyone has had this happen? my poor baby!