Allergic to "cillins"

Last night we took Elijah to the after hours clinic because he looked like he had chicken pox, we found out he's allergic to the "cillin" medications, he's been taking amoxicillin for an ear infection. Has anyone had a child with an allergic reaction to the "cillin" medications? How long does it take for it to start getting better, I have given him one dose of benadryl since it looked worse when he woke up, and as directed by the doctor I have to give it to him every six hours. It's 2 and 1/2 hours until his next dose and he looks worse than this morning. I can tell it bugs him because it's itchy, I'm just curious as to how long after I start giving him the benadryl that it will actually start looking better...