So, I broke out in hives about six to seven

Hi. I have a six year old son who is having a severe reaction to omnicef. I told the doctor he was allergic to penicillian, and she put him on this medication over a week ago. 24 hours after the last dose, he suddenly broke out in hives. I took him to his doctor and she gave him prednizone and told me to keep him dosed with benedryl, and claritin. I have also given him several collidal oatmeal baths and kept him covered with calamine lotion. His hives are getting so much worse, they look like large donuts with white ridges and deep purple centers. The hives range from dime size to five inches across. He suddenly started complaining of stomach pain and began vomiting about thirty minutes ago. Should I take him into urgent care, or is this typical? He is not running a fever, and was drinking only water since dinner last night. His stomach is hurting, and he is very lethargic, more-so than I expect from benedryl. He is responsive, just out of it. What should I do?

Hi, I noticed that when I had a cold I took Nyquil I had an allergic reaction I broke out in hives. I suffer from Sinuses and I took Alka-Seltzer plus for severe allergy sinus congestion & headache and also broke out in hives. Could it be the Acetaminophen I am allergic to?