Please help! My cat is vomiting, I don't know why?

My cat is 6 years old, and 2 days ago he threw up a hairball, then again today. And just now he threw up what appeared to be bile (a yellowish liquid). He doesn't seem his happy and affectionate self. He seems more tired, yet he was purring after vomiting the bile once I started petting him. He doesn't seem to have a temperature, I checked his ears. However, I noticed sleep in his eyes, and he was generally acting a bit out of the ordinary. Does this sound serious? Should I bring him to the vet in the morning, or does it sound somewhat normal? Please help, I'm worried!