Why is my cat throwing up so much?

Since yesterday morning, my cat (who is 2 and a half) has been throwing up. I haven't been able to watch her all the time, thanks to school, but I've seen her to it about five, maybe six times. Sometimes it's food and sometimes it's just bile, like maybe if she's been drinking milk or water. We're taking her to the vet in the morning, but I'm wondering if anyone knows what this might be. She hasn't been acting any differently, but she's never been sick before and I'm worried.