What is wrong with my dog?

Suddenly at night my dog started vomiting,breathing hard and i thought it was his normal gastritis�symptoms, but later that day he lost his appetite,most of the time he is lying and when we go for a walk,he is not eager to walk at all,he is drinking a lot of water and instantly vomits it all up. He is like that for couple of days now. And today on top of that he had diarrhea. I don`t want to think that he is dying... why should he die,he is only 4 years old! =((((((((((((((((((((((((((