Wow this is weird, my papillon merlin just had the SAME THING. he is 2 years old and he is usually crated at night next to the bed, well three days ago at like 1am he started screaming so i let him out and he pooped, loose like you described, then went back to bed, then again at 3 am the same, and at 5am, and then i had to go to work!
during the day he was fine!
Usually with this i would recommend holding his food for 24 hours and let his system settle a little, i cant do that with merlin cos he is so small and they can get hypoglycemic if not fed.
So I took him to the vet yesturday and he doesnt have any "friends", but is on an intestinal antibiotic and was fine last night.
I would hold his food for one day and give him a couple of spoonfuls of non fat plain yogurt with active bacteria in the morning and at night, and let him have access to water. Let his insides rest and then go back to normal feedings the next day, and see what happens. If it does start again go to the vet cos he may have a bad bug in his tummy.
The good news is none of the other dogs got it. But if he is consistantly good and then this starts to happen, it usually is a medical problem.
GOOD LUCK!