I'd consult your doctor. See if it is something the nurses phone line can help you with (or however your pedi works). They would have a better idea I bet. Does he have a fever with the loose bm? If so then the fact that this last diaper was not much at all may be a sign of dehydration and he's had diarrhea. Does he have tears when he cries? Are his diapers wet as well (hard to tell when they are poopy but...) I think you may want to watch for him to be dehydrated. As for the cause, could be any number of things and yes, milk allergy could be one of them. Take him off milk for a couple of days and see if his disposition changes, and hopefully bm improves... He'll be okay for 2-3 days without milk just so you can see. Or go back to the formula for those few days. Mix it in his food so he doesn't lose the milk that his body needs.
Teething is capable of amazing things as well. Especially if its really 8 coming in all at once... I know my little one had 4 at a time and all kinds of stuff would happen including lose stools... Poor babies!!!
Contact your doc and see what he says. He'll know right away if its signs of a virus or infection that's floating around and would want to see him in that case.
Hope that helps, let me know if you have more questions! I'm no professional but I'm glad to help! Congrats on the bun in the oven too! :o)