Our Puppies Are Four Weeks Old And Seem To Have A Skin Infection, Took Them To The Vet Twice Now, What Could It Be?

They come up like little blisters, and then rupture and it creates a large hole in the skin, the skin become crusty looking and then comes off along with the hair, the vet put them on an antibiotic and a steroid. She did a ringworm test but it was negative. They do not seem to be in any pain, and they are eating very well.