My puppy was not breathing at birth but began and is doing well except his leg was a little swollen at birth and has gotten much worse he is now limping. What could cause this

Perhaps his leg was injured in the birthing process, perhaps it's been injured.

How old is the pup? What does the Vet say?

You need to take your pup to the vet. It could be a birth defect or their could be a clot inside causing the leg to become swollen.

Only a vet will be able to give you answers and solutions.

Baby puppies crash and burn very fast. Don't wait around for an answer from this site.. Go to the vet now... at the very least, give them a call.