Hello & welcome, I am Dr. B, a licensed veterinarian & I would like to help you with your wee one today.
Now I have to say that I am very concerned about Annie. The change in her eating and lack of grooming are major red flags that she has mouth discomfort. Common causes for this include dental disease, gingivitis, rotten teeth, ulcers (from kidney disease or viral infections) and oral tumors (which can be ulcerative and hide in places like under the tongue).
In this case, I am very sorry to say that there is little that you will be able to do at home. The only real support you can offer is to feed her a more nutrient dense diet so she can get more nutrition even if she cannot eat much. In regards ***** ***** of calorically dense diets, you can get Hills AD or Royal Canin Recovery diet over the counter at her vets. As well, with her struggles, she may benefit from a liquid diet like Catsure or Clinicare. Finally, in a pinch, you can try pate style kitten food. The pate versions tend to be easier for cats with sore mouths to eat and again kitten food has more calories/fat/protein then her usual diet will. And at least these will help you slow or even stop that weight loss.
Finally, since you noted financial concerns, I would note that if you are on any benefits then you may be able to get her seen at the PDSA (HERE). Otherwise, even if you do not meet PDSA requirements, then the RSPCA (HERE) can help. They do have some hospitals she can be taken directly to or some branches will help with costs via their own contracted private vet. So, do look into those options since her signs are highly suggestive of mouth pain and we don't want to leave her in discomfort with this.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
Dr. B.
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