What is this red rash all over my puppy's body?

This morning when i took my puppy for his morning walk, i got home and noticed that his belly is completely red!!! Also between his legs, neck and even the ears and around his eyes!!!! I took him to the vet and he got a cortisone shot and they gave me medicine for home. But so far, nothing. He still has the rash and he's just laying down. Should i give him childrens benadryl? Like 1ml?

No do not give him anything without your vet's consent. It might interact with what your vet gave him. The rash will not magically go away in a few hours even with meds. Sometimes it will even look worse before it gets better. It may take a few days to work. I would see the vet again on Tuesday if you haven't seen any improvement.

A cortisone shot is only for itching, not for an allergy. The benadryl would help with the allergy which is most likely what it is. I know children's benadryl is safe for dogs. But I would definitely ask the vet about it first, to make sure that the ingredients won't react with each other!!