If you haven't done a FULL vet check (full bloodwork and everything), have that done ASAP. Even if nothing appears to be wrong, stuff like blood sugar, thyroid, and lyme's can cause those problems.
That said, are you positive it happened as quickly as you thought he did? Many people (even trainers) have difficulty noticing some of the warning signs. Not every dog is growing to growl, stop playing with other dogs, or act flamboyantly stand-offish before they become DA. For example, before my JRT gets into a fight (a full blown trying-to-kill) whatever dog he has been playing with, he will repeatedly turn his head to the side instead of looking at them straight on, his barks changes pitch *slightly* into a higher-pitched bark, and he'll straighten his front legs.
"My favorite color is green, green like newly cut grass. When it comes to green with envy, though, you can stick it up your @ss!" ~ Grammy