I know that my rat is sick but I don't know what with...?

My rat Arya is very sick and I know it. She came down with it very fast, before I could even realize that something was really wrong. I'm going to take her to the vet tomorrow but the vets in my area aren't very familiar with rats so I want to gather as much information on what this could possibly be before I go in. I've looked all over but I can't seem to find anything that isn't Myco, upper respiratory, bumble foot or tumors and she doesn't fit any of these. I have 6 other rats and have never had this happen before and none of them have caught this from her so I have no idea what this is. Please someone help before its too late!

Symptoms.

Fluffed up all the time.

Lethargic

Stiff gait

Unusual sleep patterns

Depression

squeaks when picked up (this just started today)

still eating but noticeably less

drinking normally

spends most of her time curled in a ball

She just seems sad and not herself...I just hope it's not too late.