My cat has vomited for the second time this week. He is clearly retching and heaving before he vomits, and his vomit was a small amount of brown/dark green liquid mush. There was no blood or objects. I have already done a little research on the possible causes, but I cannot seem to pinpoint what is actually causing my cat to vomit. I will be taking him to the vet soon, but I would like an idea of what is going on beforehand. Thank you for any help!