I had an identical problem with AJ. Vet tested his stool, no bacteria. He ate and drank normally, energy level was not affected. However: The day before this started, I took him in to a PetSmart to have his nails trimmed. He was so upset by this, he crapped as soon as we were out of the store. Everything was fine until the following day and the diarrhea cycle started. It was the same thing…poop the runs, go about 15 feet and squat again, then go another 15 feet and squat...each time straining like there was something else in there. Vet called it Idiosyncratic Diarrhea which basically meant he had a case of the runs for no apparent reason.
He gave AJ a shot that made him sleep soundly for a full day except when I got him out to go potty. When AJ woke up the day after the shot, he was fine. The long sleep gave his body time to reset and get back to normal. If diarrhea is allowed to continue, it will cause enough irritation in the bowel for it to begin bleeding. This, in turn, can cause existing normal bacteria to get places it isn't supposed to be, which creates a cascade of other problems.
I believe the nail trim did it. I had taken him to a groomer at a pet store about three months previous to this. I had to leave him there for about an hour so they could groom him. When I returned for him, he was so anxious to get out of there he climbed over the half door at the counter. I think the memory of being left in, basically, a dog prison and the smells that went with it were enough to set him off. We haven't been to a store groomer since. Haven't had any diarrhea lasting more than one poop since either.
(Added text) AJ always either throws up or gets the runs from pork. He does not get pig ears because of this. Besides…I picked a load up at a pig ear factory. No way I'd consider that edible for a dog after what I saw there.