I Have A 15 Mo Boy Whom Woke Up Randomly

I have a 15 mo boy whom woke up randomly in the night and threw up what looked like cottage cheese. He has woken up two more times so far with the same thing but not as much coming out as the first time and it seems to be more yellow now. He doesnt seem to have a fever or anything else for that matter and it seems to be worse when he tries to lay straight down. What could be causing this?