Hiding cat

So I have this cat. She's a 6 year old calico. I've had her most of her life, I adopted her from the humane society when she was still a kitten. Well she's always been a good cat, not too social, only if it's people she really knows, but that's how alot of cats are. Well recently she's started to just hide away. She stay's upstairs in my mom's room all day everyday, and if you go up to see her she hides herself underneath the bed. If you pull her out to show her some attention she'll rub up against you and start purring, which is good, but if you carry her downstairs and shut the door leading upstairs, and set her down, she just cowers. We also used to have a Shetland Sheepdog that she's known ever since we got her, but in December we had to put her down, she was 15. Then we got a couple of rats and she was still fine, but then one of the rats died and that was when she started hiding. Now both rats are gone and she still remains upstairs and out of sight. I've disscussed this with some people and no ones really said anything except that maybe shes scared and confused because everything is leaving. But we also have another cat as a companion and he's fine, but she still wont budge. Any ideas??

The sudden change in behavior may be a sign of illness. Please consult your veterinarian.

I have an appointment already set up for next week because it is really starting to worry me. I'm not an expert, and I know that if she is sick it could be more than skin deep, but her behavoir is the only thing that's changed. She's eating the same amount, no bathroom problems, no puking, her nose isn't dripping, her pupils, her heart, everything physical is fine, which is why I made a post, to see if anyone had ideas of what else it could possibly be. I'm deffinantly taking her to the vet next week though.