Why did my cat throw up three times today?

She vomited a clear liquid with a thicker green liquid in the center of the vomit puddle. I suppose that was stomach acid. She didnt vomit any timothy hay from my guinea pig's hay supply. I checked because she loves chewing on hay. She did vomit some of her kitty food [the third vomit], and the pellets looked intact [in whole pieces]. She eats store brand kitty food.