Vomiting in cats may be caused by various factors. She may be eating too quickly, food allergies are possible, intestinal parasites may be present, or hairballs may be the culprit. http://www.sniksnak.com/cathealth/vomiting.html
It's possible that your cat is allergic to some of the ingredients in her food, especially if it contains corn, gluten, wheat, soy, dairy products, and any artificial colors, flavors and/or preservatives, as these are common allergens to cats. Even if your cat has been eating the same brand of food for a while, manufacturers change formulations, and a new ingredient may be causing a problem for your cat, or she has developed a sudden allergic reaction to an existing ingredient. If you'd like to try a well-recommended premium cat food in the future, some good brands to consider are Wellness, Innova, EVO, Felidae, Newman's Own Organic, and Natural Balance. These do not contain any grain fillers or artificial preservatives. http://www.sniksnak.com/cathealth/allergies.html
It's also possible that she may have a problem with acid reflux. If she hasn't eaten for a while, an empty stomach produces acid; if she eats too quickly, the acid produces nausea and the food hits the stomach, causing vomiting almost immediately, or shortly thereafter. Acid reflux can also be present, even if she doesn't go a long time between meals. The best way to minimize acid reflux is to have your cat eat more frequent, smaller meals. Having dry food available at all times, along with fresh water, is always best. Sometimes, 1/4-1 tablet of Pepcid AC daily, will help the situation, but please don't give any over the counter medication to your cat, before consulting with your vet. http://www.petplace.com/drug-library/famotidine-pepcid/page1.aspx
Raising up your cat's food/water dishes to chest level, should help, as this aids digestion. Eating too quickly can also contribute to vomiting right after eating, or shortly thereafter; to avoid this, place a new, washed, unused ping-pong or golf ball in the middle of the bowl of dry food, to make her eat 'around' it. For canned food, serve on a flat plate, mashed down with a fork, so it's not 'mounded up' in the middle.
If you feel that your cat might be ingesting a lot of fur during self-grooming, and hairballs may be contributing to this problem, combing/brushing several times daily, will help. Hairball remedies like Petromalt and Laxatone, available at stores, might be a good idea to offer daily, or a few times a week. Putting a one-inch strip of the hairball remedy gel on your cat's foreleg, right above the paw, will cause her to lick it off, if she won't take it directly from your finger. Also, if it's not actually a hairball causing her to vomit, some fur might be 'tickling' her throat, causing her to gag, and then throw up.
Intestinal viral or bacterial infection and ingestion of a toxic or non-food item, is always a possibility (do you have any plants/flowers, live or dried, which can be nibbled?) so the vet visit will help determine if there are any toxins present through routine blood work. Intestinal parasites like worms and protozoans (giardia, coccidia) can also cause digestive problems. A stool sample should be analyzed, to discount the possibility of intestinal parasites. However, there are a variety of other intestinal conditions which may also cause vomiting. http://www.catsofaustralia.com/vomiting-cats.htm http://www.sniksnak.com/cathealth/worms.html http://www.sniksnak.com/cathealth/coccidiosis.html http://www.sniksnak.com/cathealth/giardiasis.html
Stress can also cause digestive problems. A cat will become stressed if there are any changes made to her environment or schedule. Feliway/Comfort Zone for Cats is a cat appeasing pheromone which can help to relax stressed cats. It comes in both a spray and plug-in diffuser. The diffuser plugs into an electrical outlet like an air freshener, but has no scent to humans. For more information, please see: http://www.catfaeries.com/feliway.html
Major pet supply stores carry Feliway, you can also order it online, and some vet offices carry it.
I hope some of my suggestions prove helpful and your cat will vomit less frequently, in the future. If this continues, it would be a good idea to have her evaluated by the vet, ASAP, and bring a fresh fecal sample with you, for analysis.