Thick skin/scab with fur on it, is sloughing off, and new skin has no fur???

Hi,
My cat has a spot with thick skin/scab that has fur on it on her hind leg (kind of round, not a circle). It is sloughing off, and the new skin has no fur. She had a smaller one (linear, not round) on her other hind leg at the same time which has healed, no fur on the new skin.
My vet doesn't know what it is. He thinks she might have been bitten by a spider or it is a scrape. He thinks she has low immune system. She did have mastitis that look along time to heal, 3 surgeries. She tested negative for Feline Leuk and AIDS.
She had vaccinations and antibiotics less than a week before I saw the sores. I mentioned to him it appears to be at the vaccination sites, but he didn't think it was relavent. He said the sores were local, superficial, and injections are intramuscular.

Does anybody have any ideas??

Thanks

Do you remember a wound on her? Was she licking/biting at the injection sites? I do think it's relevant, by the way, coincidences like tend not to happen. I've not had a cat lose fur from an injection, but I think some of our members have.

And if her immune system is low, as the vet suspects, you need to really be more vigilant. Regularly (as in daily) check her for any sort of cut or sore and make sure to treat it to decrease chance of infection.

I don't remember any scratch or wound on her at all. She had been confined in a bathroom wearing an E-collar recovering from her surgeries for about 4 weeks. When the E-collar came off, she did clean herself a lot, maybe she was licking the areas. It was a few days after the collar came off that I noticed the area in question. She was still confined to the bathroom.

My vet also suggested as you did, to inspect her daily.

Thanks