Cat had blood on rear after using litter box?

I adopted my cat from the shelter a few years ago. She had acquired the feline version of the herpes virus in her eye from living at the shelter, and has had some issues because of it: Diarrhea, sensitive stomach, eye problems...etc. yesterday, I notice she threw up quite a few times... but this is not unusual for her. But I did notice that it looked like she coughed up two hairballs. However, after that she seemed fine. Today she seems normal except I noticed after she went upstairs to use her liter box she had some bright red blood on her anus. I ran upstairs to check her box, but she has already buried it and I couldn't find any blood in the liter box. She isn't acting sick and seems to be eating normal. I'm really worried about her, and I don't know how soon I'll be able to get her into the vet. Is this serious? I had a pet die tragically two years ago, and I don't know if I could take another one dieing on me. I'm so worried. Any ideas what could be wrong?