I don't suppose it could have been possible that that string toy had more than string in it ?
Besides, you'd be surprised what a dog can get into in the smallest of confined areas. I mentioned in another thread that my dog picked up / ate something that made her sick while she was being watched like a hawk. You'd have to be made of eyes. My dog has been known to bite her toenails, anything I supposed could cause a little wince on the way out.
It's odd though, and I think untypical, for your dog to have that extreme of a reaction to even a sore butt. My dog (after the mysterious edible and resulting sicklies above), was so sore after being sick that she cried and cried while the vet took her temp and checked her butt, but never while she had been pooping. She also once had badly impacted anal glands, with no wince's but butt-scooting ...the wincing and screeching was at the vet
. The only time she's ever done the travelling poop parade is if she was really sick, or got a really good scare.
Is his tail ok ? Could the movement req'd to poop be twinging the tail somehow ? No chance anyone walked on his tail by accident ?
poor baby, it's always scary when they do the screech, even when it's at the vet. Can't hurt to do a follow up call to vet to ask for advice, see if this could be a vaccination issue.