Thank you Emily,
Now Cinders signs are quite vague and she isn't giving you much hint to what is specifically amiss for her. Her shaking, reluctance to jump, refusing to use the box (often a sign that doing so would be sore for them), refusal to play, and subdued nature all hint at a possible fall or trauma causing soreness for her. That said, we'd not expect that to cause the decline in appetite unless she has had a proper wee fall when you weren't looking. So, while I am very glad she has no belly discomfort (since the ingestion of non-edible items is always a concern with poor appetites in young cats), we would also want to keep a close eye for any hints of GI upset for her as well.
With all this in mind, I would advise thoroughly examining her now. Since she isn't lame, I'd not expect anything sinister (ie fractures, severe muscle swellig, etc) but we want to check for any areas of discomfort for her. If you find any, these can warm compressed to give her muscles some relief. We, of course, don't want to give any human pain relief (since they are toxic to cats) but keeping the muscles warm and comfortable will help with any soreness.Further to this, one support that you could use at home to address soreness is Omega 3 & 6 fish oils (EPA/DHA). These are actually a natural anti-inflammatory agent that people use to ease discomfort in inflamed muscles. So, these could be used at this stage as well. A proper daily dose for her size would be 20mg EPA/DHA per pound of her body weight. So, this too could be a consideration for her . As well, if she isn't eating well for us, you can also consider putting her onto a light/easily digestible diet. Examples of this would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used in cases of gastroenteritis (ie Hill’s I/D or Royal Canin’s sensitivity). Ideally, these should be fed as small frequent meals instead of large ones just in case there is an underlying GI upset here too.
Now since her signs are more suggestive of a sore kitty instead of an ill one, we may not need to address any nausea for her lack of appetite. Still just in case you do think she is nauseous or she isn't keen on those light diet options, I would note that you can try her on an antacid to make sure her stomach is settled. There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the two I tend to recommend are Pepcid (More Info/Dose) or Zantac (More Info/Dose).We tend to give these 30 minutes before offering food to give it time to be absorbed. And of course, we’d want to check with her vet first if she does have any pre-existing health issues or is on any medications you haven’t mentioned.
Overall, Cinders signs are vague but hint more at a sore kitty then a sick kitty. Therefore, I'd advise the above approach for her while monitoring. If you do find a sore point in your examination of her, then they can be warm compressed and monitored until Monday. If it is just a bruise from a fall, this should settle over the next 24-48 hours; but if she remains sore or develops any GI related signs then we'd want her to be checked over by her vet on Monday to make sure there is nothing else brewing and nothing in her stomach that shouldn't be.
Please take care,