Discussion: Guess

I'll plump for strain/sprain with associated inflammation - hence nothing much until you'd had time to irritate it for 15mins or so and increase the swelling aka inflammation.

That would seem to make sense - most likely something fairly deep that isn't structural (even when it's at its most painful I can walk freely) but is swelling in a place that causes trouble. Hopefully you're right as that shouldn't take too long to settle down (at least compared with possible alternatives).