My cat has a red hairless patch with blood. Is it serious or an emergency?

Hi there! What you're describing sounds more likely to be a skin problem of some kind (allergies, etc.) vs. an actual wound or something more serious, although of course it's always hard to say without being able to see her in person.

It does sound like this is something that should be seen by a vet, since skin issues in cats that are severe enough to cause hair loss and raw spots will virtually always need prescription meds to treat effectively. But if she's acting normal otherwise (eating and drinking, relatively active, no other obvious symptoms), then I wouldn't consider it to be an emergency. It should be okay to call your regular vet's office and get an appointment scheduled for her when you're able to, hopefully within the next week or two. :)