Allergic reaction/rash to shellfish

My 7 month old daughter had her first bites shrimp today while we were out for lunch. We typically feed her what we eat so we didn’t think anything of it. While changing her diaper this evening ( about 2 hours after lunch) I noticed a bright red bumpy rash all over her legs, back and arms. I went immediately to the drug store to find infant benadryl but could only find Children’s Benadryl which on the box said not for children under 2. She has no fever just a rash but I’m not sure what to do. Should I take her to the ER? Can she have any other allergy medication?