My dog i dont know whats wrong cont....?

my dog has not eaten that much today he ate very little in the morning and very little tonight, tonight i think he has a stomach ache, but this morning idk. he ate a lot yesterday he was great yesterday. but i dont think eating is the problem, because he usually goes through this, eating little one day and the other day just pig out. but yeah and he played in the pool for about 45 mins today and now he is throwing up. he threw up 9 times in past two hours. nothing but water, and saliva. he is 5 months old, no history of health problems. he is a yellow lab.can someone please help me im freaking out.