I really need some advice right now as I don't know what else I can do. I've been looking after a few feral cats for the past 4 years and they have all become very comfortable around me, at first I couldn't get close to any of them but with time they became affectionate and friendly. However, on Monday i noticed that one of the feral cats was behaving very strange. She seemed extremely scared and wouldn't eat, she would come up to the food, try to take a bite but would then run off and hide. Since then she has only gotten worse and now won't come anywhere near me, if i try to go up to her with food she will run off. I have tried feeding her some smelly fish like tuna but she doesn't seem interested in food now. She might have a problem with her mouth but I don't know why she seems so scared

I have called up my vets who are also an animal rescue charity but they haven't been very helpful, I understand it must be a busy period for them but they know this is urgent as this poor cat hasn't ate for the past week. They said they will try to get someone to come down to trap her and will get back to me about when they can do this but couldn't say when. I have also tried trapping this cat myself but it's almost impossible as she doesn't seem interested in food and keeps hiding under cars. Every time i chase up the vets they tell me that I have to wait for someone to get back to me, but it's extremely frustrating and I don't want her to be suffering any longer.
I'm reluctant to call the RSPCA as I've had some bad experiences with them in the past and they'll probably take even longer to help

Sorry this is long and thank you in advance for any advice.