Update - I think we've worked it out!!
I was cuddling on the couch with Poppy yesterday evening, and suddenly felt a large hard lump on her side, sort of egg-sized. You know how your blood runs cold in those moments? Within 30 seconds the full scenario had run through my head, with a dreadful diagnosis, a horrible few months and a tragic end...

Then I realized the lump was at the site of the rabies vaccination. Phew...
Poor Pops. She almost certainly was feeling under the weather on Sunday as a reaction to the rabies vaccination (
@M.F. - you were right, I think!). I thought we'd left enough time between the vaccination and the training day (five days), but another time I must remember to leave a full week, at least. Now we feel like bad doggie owners...
She does seem much better now, was running around like a loon on our morning walk...