Does she have any other symptoms, like diarrhea, vomiting, hiding? I'm no vet, but if it was my cat I would be sick with worry and would take the cat to a veterinarian ASAP. The red spot with missing fur could be ringworm, which is contagious and quite different to get rid of, will need the right medication to threat it. I had a kitten with a weak immune system that kept getting ringworm. It took months of treatment and home made meals for it to clear. The red around bum could be a number of things. Something is causing a lot of inflammation, irritation and discomfort. Could be parasites, a rash, a food allergy. Only way to know what it is and to get your cat the right treatment is to take her to an animal clinic. Good luck