There's a cat that "lives" in our garden while her owners live a few doors down. They simply have too many animals and keep getting more. I think she's been pushed out. She's a very feeble cat and struggles getting on and off the sofa etc. She used to be very shy and didn't really know how to play or interact like a normal cat (I'm a bit of an animal behaviour geek) but over the past few of months she's been looking and acting SO much better since we've been feeding, brushing and treating her for fleas and what not. She seemed to acquire this strange habit of once she's eaten she'll run into the garden for no particular reason.
However, recently she's been acting really strange. She doesn't want to come in, she won't eat unless her food is outside and she lost her voice like she has laryngitis. Now she seems to have clammed up completely and barely meows at all even though her voice is almost back.
I used to be a vet nurse and to be honest, I'm worried. Her glands are up and she's losing weight but she's not my cat and I can't afford to take her to the vet myself. I contacted the PDSA to see if they could help but because she's not MY cat, they can't contribute to the bills. They told me to phone the RSPCA but the rest of the animals looks fairly well looked after. Even if they came here to see her, she seems to disappear most of the day now and she never used to. Another reason I'm reluctant to get the RSPCA round is because I'm not sure if her owners know that she spends most of her time here.
I just don't know what's going on in that little head of hers. What could cause behaviour like this? I've never seen or heard anything like it.

Could she possibly have kittens hidden somewhere?

It did cross my mind but I'm fairly sure she's an old cat and if she is a mother, she's not overly attentive. She still spends some time sunbathing in the greenhouse like she used to. I'll see if she's lactating.

I'd still get the RSPCA involved. Even if the other animals look good, the owners clearly aren't observing the problems with this one. She needs medical attention. Or, you could talk to the owners and tell them what you've observed, and ask them to get her to the vet. Good luck...