Thank you for the additional information.
When we see a young cat that is vomiting, not eating and not passing stool our first concern is a intestinal blockage from something they ate that is either to big to pass through the digestive tract or some type of string that has become wrapped around the intestines. In some cases the brown vomit can actually be fecal matter that is coming up since it can not pass . Click here for more on this
Other possible causes could be
* bacterial infection in the digestive tract
* Ingesting something toxic including plants and human medications.
Cats unlike dogs will not stop eating unless they are sick,in pain or severely stressed. When a cat stops eating the body burns fat for energy only a cats liver can not process this fat resulting in a condition called hepatic lipidosis which can lead to liver failure so it is very important that we figure out what is causing her to not eat.
The best thing would be to have her seen at the vet today even if this means going to the ER. If this is not possible you can try 1/4 of a 10mg Pepcid AC and then wait 30 minutes then offer bland food such as boiled chicken or chicken baby food ( no onions or garlic in the ingredients) If she does not eat or has any additional vomiting then she needs to be seen as soon as you can.
I hope this helps give you some direction and she feels better soon. Please reply back if you have additional questions or concerns and I will be happy to continue.