Acute vomiting can be caused by many different things in cats. The list includes (but is not limited to):
Gastrointestinal obstruction with a foreign body (such as a string, or a piece of a toy) Toxic ingestion
Gastrointestinal infectious disease (viral, bacterial, etc..)Kidney
Differentiating these illnesses requires a physical examination by a veterinarian and blood and urine laboratory testing, plus abdominal X-rays.
Since your kitty has only vomited a couple of times a day and does not yet appear to be showing signs of dehydration, I do not think that you need to get her into an emergency clinic tonight. You should just keep your eye on her for the evening. Once she has not had any vomitting for 6 hours, try to encourage her to drink some water. If she will not drink from the dish, then draw some water up into a syringe or medicine dropper and try to squirt a little bit into the corner of her mouth. If she keeps this down, then offer her another few tablespoons a few hours later.
Because this has now been going on for three days, and she is acting very lethargic, she really does need to see the veterinarian. Simple dietary indiscretions or minor gastrointestinal viruses usually do not last this long. Really, there is likely something more serious than this going on that needs attention.
If you check her gums and they are getting sticky or dry, or if you should notice that her gums are getting very pale, or she begins vomitting intractably, then she is going to need to see a veterinarian right away.
I am attaching a client information handout that I use in my practice concerning acute vomiting. I hope that you find it useful.
I hope that this information is of help to you, and I wish you the best of luck with your kitty. Please let me know if I can be of further assistance.