It sounds like it could be a very mild case of shivers. The two conditions are related, as I understand it, but stringhalt describes the symptoms when they happen in movement, and shivers when the horse is standing still. Shivers is also usually evident when the horse backs up - not necessarily every time, but frequently.
However, if this was a one-off incident, and he hasn't show the same symptoms before or since, it might have been something else completely - a "trapped nerve" for example - that we sometimes get too and often resolves itself very quickly. The fact he really didn't want to pick the foot up in the first place suggests something more pain related. My Bigsy has a mild case of shivers - but he picks his feet up fine - he just picks up really high and shakes it for a few second with the hind legs. He doesn't seem distressed by it at all - he doesn't really seem to know it's happening - or rather that it's anything unusual!
Your vet is the one to say what it is. Shivers/stringhalt can be serious - but there are also thousands of horses who have these conditions and continue
normal work without any problems - so it's very individual thing!