What could possibly be wrong with my puppy?

I woke up this morning and heard my puppy screaming. It frightened me so I let him in my room and he started to gag for a long period of time. Then he spit up something that kind of looked like his food that wasn't digested right. So I take him back out to the other room and i see that he has been throwing up all night. Description of the vomit: First piles are chunky with what looks like undigested dog food in there. The more recent areas of puke are kind of yellow and slimy looking. He did eat out of my bowl of Cocoa Pebbles when I set it down yesterday could chocolate be the issue? Anything anybody knows about this it would definitely be appreciated. I ask only serious answers please.