Read this http://www.sniksnak.com/cathealth/vomiting.html

Conclusion, take the cat to a vet now.

I'm a new poster here, but thought I could give some advice since I had a
similar experience with my kitty recently. Jasper started vomiting here and
there for a couple of days. He then stopped eating his food. We took him
into the emergency vet clinic because we knew if we let it go, things could
get much worse... not just healthwise, but also the potential high vet
bills if it developed into something more serious. It turned out that he
had a bacterial infection and he was diagnosed with gastritis. He had to
take a five-day course of oral antibiotics and anti-nausea medicine. He's
now back to normal, eating well and playing like he always has.

I would definitely take your cat to the veterinarian. It could be something
really minor, but it's better safe than sorry.

~miette