Poor Shelby. My fur bub has done this a couple of times, last time was when I was bathing DS for the first time by myself, and home alone.
He also allows us to touch it etc and it feels fine. Not sure what sort of pup you have, but I have a little one who gets overly excited and "pulls a muscle" in his back leg. We normally give him a treat that will take him some time to get through, so he can lay day quietly for a while.
Check for burrs as well caught on his foot.

Have you got a 24hour vet close by that you can call? I know we have one who we have called before, and they kind of triaged us over the phone to see if it was something that could wait til the next morning, or if he needed to be taken in straight away.

I'm sure he will be ok, but that doesn't stop you from worrying I know!