Thanks everyone. I have an appointment for her next Wednesday at the low cost pet clinic. Hopefully, she'll get a clean bill of health.

Out of the four, 2 are fixed (the male and the mama cat). The other 2 don't go out at all, but I plan to get them both fixed as soon as I get the extra $50 to get each of them done.

I haven't changed litter or where the box was. I'm thinking about getting another box just for her to use. I don't know how I'll keep the other cats out, though. LOL!

Thanks again for your help, everyone. I'll post here after her doctor appointment and let you know how it went.