Does she have a flea collar on?? If the sores go all the way around, my suspicion is that it's caused either from an imbedded collar (she could've had the collar on from the time she was a kitten and got to big for it), or an allergic reation to a flea collar. You are wise to get her to a vet as soon as possible.

Once she gets a clean bill of health, keep her separated from your other cats. Take a towel and rub her down and let your cats small her scent, and vice versa.

You are doing a great thing. I'm glad you're going to take her in. I hope she's okay.

Good luck and please keep us posted.