Child throwing up stomach acid?

I have a 5 year old and when he woke up this morning at 6 he started coughing and coughed til he threw up, when he threw up all it contained was stomach acid/flem. He did this twice within an hour of each other and then hasnt got sick again all day. Its now 5 p.m. and he has ate all day long and not took a break from playing! Can anyone tell me what possibly caused this or why he done this?