Hello, My Son Was Diagnosed With Strep On Tuesday, September

Hello, my son was diagnosed with strep on Tuesday, September 15th and put on Amoxicillin 875mg 1 tablet 2x a day. On Thursday, September 17th he was complaining of back pain and his fever was up to 104., we took him to the ER complained. While there they did a lung xray, which came back good and a urine analysis. He was diagnosed with a UTI and put on Cephalexin 500 mg 1 tablet 2x a day. He is still complaining that is throat hurts and today he has broken out in a rash all over his body; he is not presenting with a fever. Is it possible that he is having a reaction to one of or the combination of both antibiotics?