Anyone know what this is? (Vet said probably caused by injury, but is getting worse)

This is between her tummy and groin. Took her to the vet on Day 3 where they said it was probably caused by an injury (told it was definitely not ringworm). Was given Isaderm to put on it. It's now Day 10 and looks worse. Surely if it was an injury it wouldn't be getting worse? She has no other symptoms. Planning to take her back to the vet hopefully on Monday, as they said it would likely be gone by now.