Justin Bieber -- Let's Talk Vomit

Here it is ... straight from Justin Bieber's mouth ... a full explanation why the pop superstar heaved his guts out on stage at a concert this weekend -- and it all comes down to moo juice.

Bieber just called in to "TMZ Live" and said he had been throwing down whole milk and spaghetti bolognese right before his concert in Phoenix on Saturday ... and it turned his guts into a ticking puke bomb.