Why Is My Kitten Throwing Up Clear Liquid?

My kitten is throwing up clear liquid, he eats and drinks his food but does not vomit. He has been doing this for about 2 weeks now? I have taken him to the vet twice they did blood work and said everything was fine. What could be causing this? We are not sure what kind of cat it is just know that its a small black one with white feet on his back side please help!