You likely pulled it earlier than you think. It was after your muscles cooled down that it was noticeable.

You need to stretch more, mate. From a gym brother to another, do some yoga or tai chi.

I've been there so many times, my hips have gone out from me sneezing, my sciatica was messed up for a while, I've put my back out by simply leaning a tiny bit towards the mirror while I washed my face (can't see without my glasses lol) I agree just do some stretching each day, but do that once you've healed and start slow! I hope you feel better soon!

Probably overworked your muscles then.