ALLERGIC REACTION!!!!?

My baby woke up with a rash yesterday and i called his dr and they said it would go away within a few weeks to give him benadryl if he was uncomfortable. He was fine all day and at midnight he woke up crying he was fussy and itchy we fell back asleep and we just woke up and he's completely swollen! His little face and eyes why is he getting worse?? Will the benadryl take this swelling away do i need to take him to the er??