She still seems to be aeting and drinking ok, and goes out to the toilet. Up until late today she semed fine and I thought that changing the cat food had done it but she was quite sick again this evening and just looks awful. She is still runing around after me as usual and was playing with my son before he went to bed but just doesnt look herself. I may e overeacting but as she has never been ill before it has taken me by surprise. She has always been very healthy and quite hardy wee cat but tonight she looks old. I am really hoping it will turn out to be nothing terrible.

I hope that you are right. I am hoping that I am worrying without real cause, I have been on google and there are a lot of posts about grass being stuck etc. The more I read the less worried I am as in any case where the animal has been really ill they have not been eating or making it to the cat tray/outside and thats not the case with mine. The sickness in itself did not worry me initially but loosing all that weight so quickly did. Thank for all your support and posts guys. Fingers crossed it will go well!

one day of being sick loads and then it tailed off and now a couple of times a day. Today only once that I know of. She goes outside a lot.

The weight loss like that is a bit weird, i have personally not seen a cat lose alot in a few days especially if their still eating.
I am 50/50 on weather she needs to be seen asap or monday, if it is a 'chronic' condition which is possible at her age then realistically 24 hours is not going to make a huge difference.
The main thing is that she is acting normal but it is difficult for us to tell, obviously it is your call weather something seems really 'off ' or not, because you know her best.
It could possibly be thyroid troubles or some irritable bowel xx

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If it came on really suddenly then it is very possible she's eaten something she shouldn't. Poisoning is unlikely though i would have thought

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Hope she got through the night ok x

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I would certainly keep her inside from now on where you can monitor the situation more closely in terms of symptoms etc. Since you mentioned a neighbour has been feeding her you need to rule this out as a possible cause- she may also have eaten something she's caught which disagreed with her!

For the moment I would just feed her a bland diet ie chicken and see if she improves.

However sudden weight loss is always a concern and it's important to find the cause even if the symptoms clear up. So definitely a vet visit tomorrow.