hurt dog

[Shawn Braquet]

just got in from working my lab and he wouldnt sit or jump like normal. Noticed a hard lump behind what I would call his elbow on his rear leg. Its definitely not on the other leg and when i push on him to sit( force him) he lays on the opposite leg as to take the weight of the other one. Tuesday when I was working him he hit a hole and barrel rolled a coupletimes but after that never acted different. then yesterday didnt work him but let him out and he didnt act different. today he wouldnt sit but would run and retrieve if i didnt make him sit. When I forced him to sit and sent him to retrieve he would slowly get up then run and retrieve. any ideas? pulled muscle? banged up and bruised? if he's doing it friday I'll bring him by the vet I guess.