Is it in both legs? If yes I'd wager it to be simply an ankle mobility issue.
If not, then it's specific and could be general inflammation or something. Try an advil and see if it helps, if it does help it's likely just inflamed from under use and nothing to serious. Yet.
Then go get it checked out. Injuries are way to hard to pinpoint and asses online.
"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents... some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the light into the peace and safety of a new Dark Age."
-H.P. Lovecraft