My apologies that I didn't have a chance to respond to you before I had to be away (had to see one of my own poorly patients). Still I have had a read of you answers and have been considering Mylie's situation.
Now as I noted before, lethargy is a non-specific sign of cats feeling unwell. Essentially, depressed activity and increased sleeping is a cat version of us staying in our pajamas all day when we are feeling a bit off. Now from your answers, I have to say that Mylie is not giving us much to go on. That said, this does mean that we can rule out a few issues. The most important is that there has been no sign of her getting into anything she should not (since that would be our main urgent concerns). As well, if she has no respiratory signs, then cat flu type issues would be less likely. A lack of vomiting or diarrhea makes active GI bugs less likely; though appetite decline can be an early sign of this (as well as another non-specific sign of a kitty being unwell).
So, in this case, we do have to appreciate that at the moment supportive care and monitoring would be our main approach. If her appetite seems to be waning, then you can tempt her to eat with a light/easily digestible diet. Examples of this would be boiled chicken, 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 here (ie Hill’s I/D or Royal Canin’s sensitivity.) These can be fed as small frequent meals and are a good source of nutrition if we have kitties with a bit of GI upset (since they are easy to digest and well tolerated by the GI).
As well, if she isn't drinking as much as usual then it would be a good idea to check her hydration at this stage (as a baseline). To do so and make sure she is not becoming dehydrated there are a few things we can test at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether she has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. They use a big dog but it makes it easier to see and the principles are exactly the same. And this is a good way to make sure dehydration is not an issue for her (since this can make them feel poorly).
Overall, her signs at the moment are vague and non-specific. They only tell us she is feeling off but she is giving us no hints of the focus for this. This may be because her immune system is fighting a bug and doing well enough that signs are becoming apparent for us to appreciate. Or this can be the early signs of something that is brewing. In either case, I'd not be alarmed but would advise monitoring her just now. A light diet can be helpful if there is a bit of GI upset and make sure she is getting those nutrients her immune system will need. And keeping an eye on her hydration and fluid intake will ensure you don't let dehydration sneak in. Otherwise, it is a case of keeping an eye out for other signs. If she doesn't pass in 48 hours or so, then we'd want to consider following up with her vet so they can perform a full physical to give you an idea of what is sapping her energy and causing this lethargy.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
If you have any other questions, please ask me – I’ll be happy to respond. Please remember to rate my service once you have all the information you need. Thank you and hope to see you again soon! : )