Acting strange, not sure whats wrong

I just added 2 foster cats (a private arrangement) to my home zoo which already had 3 cats. They arrived monday morning and have been getting used to my place. Jane has actually been aclimating faster than the other cat, has been eating and wandering the house when she feels like it. Three weeks ago their owner was in a car accident and has since been in the hospital and unable to care for them. The woman's daughter told me that Jane has not been eating since that time, and in fact when I got the cats Jane was only skin and bones. She also had a UTI in the recent past that had been treated. She has been on Science Diet C/D. Since the cats have been here, Jane has been eating both hard and soft food and although there has been plenty of territorial hissing (nothing I didn't expect), none of the cats have tried to fight. Jane and the cat she came with have lived together for some time and were comfortable with each other. Both loved affection from me and night before last Jane slept on the bed with me.
Here's the problem: Yesterday evening Jane went into the bedroom where Lou (her friend) hides and hasn't really come out much. She has stayed in one place and wasn't interested in being picked up or even petted really last night. I didn't think much of it then, though she was just not in the mood. But this morning I'm up early and at 5am I went to check on Jane and Lou and Jane hissed even when I tried to pet her, also she hissed at Lou when Lou came near her, and she is acting decidedly unfriendly which is really not in character for her. Since she isn't interested in coming out to eat, I took in some canned food and some water for her, and she is eating. But I am worried there might be something medical going on with her for her behavior to change so suddenly.

Poor kitty all the stress from changing enviroments and being without her meowmy probably brought on another UTI. Since she really did'nt eat or drink for a while, her urine became concentrated and therefore probabably caused a blockage. She really need to get to the vet ASAP to rule out a UTI - they are nothing to play with. I hope she is ok. please let us know what the vet says.

She is probably really stressed. Take her to the vet. I agree she might have a UTI again. I will send wellness vibes. Please keep us updated.

I agree a vet visit would be a good idea. I'm also worried about the "skin & bones" part- has Jane started to gain a little weight since being with you?

You're wonderful to be taking such good care of Jane and Lou.

Please keep us updated.

I work at a veterinary clinic so I'm a little paranoid about these things. Are the cats you're fostering indoor only cats? Regardless of whether they are or not, I would definately keep them seperated from your resident cats until you get them both tested for FeLV/FIV. At the clinic where I work the test costs $29.00 per cat. Weight loss, inappetance and hiding are all symptoms of either of these viruses (as well as many other illnesses). She needs to be seen by a vet asap. Cats generally do not show symptoms until they are very, very ill. The reason being that in the wild they would be killed by predators if they did. By not getting her examined by a vet you're really putting your cats at risk if she has something that is transmittable to them.