It's not uncommon to break out in hives days after starting an antibiotic, even if you had taken it before with no side effects. This summer my 8 yo was on Amox, which he had been on before, and on the 9th day he broke out in hives and his face/lips/fingers/feet swelled terribly. He had to be on steroids and benedry for days. It took 2 weeks for the hives to go away. I googled it and was surprised to find out how common it was.