Cats butt is bald and seems unhealty

Hi, I just came back from travels today. I found my cat with a bald spot under her tail.

Its quite big and it seems like she has a wound too.

I visited the vet a few weeks ago and she told me to change her food because she may be allergic. (Small woulds from scratching in neck) previously royal canin sterilized, now hills science plan sensitive stomach and skin. Since the change her poo smells worse.

I'm seeing the vet tomorrow, but I'm so worried. Can anyone help me out?

Street cat Female Sterilized 9yo