Oh no, I'm so sorry, so upsetting.
I think my timeline is all messed up now; it seems like the collapse was before the oat+broth breakfast? Jean's comment about the cold makes sense, esp if he seems somewhat better/more mobile now. Ice on back might feel good, like Jill said. When MP's thing was all still more mysterious and leaning towards back injury, the rehab vet felt through the spine and said there's points that would make a dog collapse when pressed, if it had been an injury/swelling.
Thinking of you guys!!