It turns out Pablo was not suffering from a sprain a few weeks back. When we returned from NC, he kept trying to get at his rear paw (we thought). His suffering became worse at night (the first clue, as fleas are more active at night). It did cross my mind that he might have picked up a flea or two in NC and I checked him out, looking specifically for those telltale dots of dried blood aka flea poop. He came up clean, but perhaps the infestation was in its early days. To play it safe, I gave him his monthly dose of Frontline.
Gradually the issue with his paw resolved itself and I forgot about it–until yesterday when I was getting him ready for a bath and discovered a colony of wingless intruders at the base of his corkscrew tail. We are now on full flea patrol.