My dog is limping on his left hind leg.HELP?

hello.i have a 2 year old mix and he is limping on his left hind leg. he just started today, but im worried.He did this before,a long time ago,but after a while it stopped. Usually, he is active and playful. Now he can still walk, but always lifts that leg when he does. He is also unusually sad. he sits in the corner laying down with his leg up. i don't how this happened but he is a crazy dog. he can jump over a 4ft fence, and he does. when i pull his leg, he yelps. help me????