Thanks for the additional information. I suspect she vomited bile because her stomach is empty. Bile can be very irritating to the stomach lining and cause patients to bring it up.
As to her other symptoms, there actually could be several possible explanations for her behavior:
1. She could be running a fever.
I understand about not having a thermometer but if you manage to get your hands on one, it would be good to check her temperature (although this isn't always the easiest thing to do in a cat). Normal is between 100 and 102.5.
Of course, then we'd have to wonder why the temperature is elevated. Explanations for this could be: a bacterial infection such as an abscess (even though she doesn't have any obvious wounds on her) or a viral infection.
There is a condition in cats that is called Fever of Unknown Origin--this is exactly what it sounds like which is that we can't find an explanation for the elevated temperature. Treatment is primarily supportive which consists of fluids and drugs to reduce the fever. Most cats recover very quickly.
2. Pancreatitis can happen very suddenly; these cats just stop eating and can become lethargic. We don't have a good explanation for why this happens, either.
We do have a test for this (spec fPL) although I've come to doubt its reliability in some cases; treatment consists of fluids and pain medication (we believe the pain is why they stop eating).
3. Other systemic diseases such as kidney disease, liver disease, anemia, problems with the heart, etc. although she's a little on the young side for these. I'll just include them to be complete.
4. Ingestion of certain medications/drugs can cause this behavior but this is far more common in a dog than in a cat. If this is the case, though, then treatment will be supportive until whatever is in her system is gone...which might take about 24 hours or a little longer.
5. Some cats will just have "off" days and we're none the wiser as to why. I suspect a viral infection but can't prove it. Usually the signs last about 1-2 days, at most, though.
Since she's a little nauseous, she can be given over the counter Pepcid AC at a dose of 1/4th of a 10 mg tablet twice a day.
Maintaining hydration with water or Pedialyte is important, especially if there’s a fever although since she's only been ill for a short period of time and she's so young, this is less likely a concern than if she were a kitten or a senior citizen.
You can dribble this into the side of the mouth with an eye dropper or syringe every several hours.
If she's still behaving this way tomorrow, then a vet visit would be prudent but I wouldn't necessarily be overly concerned about her just yet.
I hope this helps. Deb