Good to hear that some other people stretch.
Just sounds like a bit of a strain to me. Especially if you did it when stretching. I think if it was a trapped nerve it would hurt a lot, all of the time rather than coming and going.
Best thing to do is to ease off on it for a few days (i.e. don't do any high steps or stretches on that leg) and it should mend itself.
Also stretching after a bath/shower is is a good way to make sure you are warm.
p.s. Dylan