I would definitely get it checked out, but don't stress yourself out too much until you get a diagnosis. Dogs can pull muscles and get sprains just like we can. He might have moved wrong or jumped and landed on it in an awkward way.
A few months ago, Miles started limping, and I was so scared (I believe I posted a forum, also!). After a few hours, he was fine, and he's been fine ever since.
In the mean time, just check in between his toes to make sure nothing is lodged in there, and press lightly on parts of his leg (he'll let you know if it hurts). |