My cat puked up blood?

I took my 7 month old kitty to the vet last night, spent 6 hundred bucks and they have no answer as to what caused my kitty to puck up large amount of nothing but bright red blood. My kitty was fine all day, playing with my other cat. All the sudden I thought he had a hair ball and he puked up up some cat food, so I cleaned it up and then he started going to the front door and puking blood. Right now I am so post to go pick him up after he has stayed all night at the ER vet with IV fluids with no answers. All test came back normal including an xray showing no object like medal, ect ect. Has anyone ever had their cat puke up blood because I thought he was going to die, but they are saying that dont know what caused this ($$$$$$$) lot of money later!!!!! Err I am frusterated! Thanks for reading!