my pup was a mad jumper. I mean, since I got him at 8 wks old, he'd leap off of any surface. turns out at 6 months old, playing with a tall 20 lbs dog I was pup sitting, hip checked my ollie (8 lbs at the time), into a wall and broke ollie's leg.
did you get an xray? I would. just to be sure. there is a very common puppy injury, right below the patella is a small bone that has growth plates on either said of it, making it suspectible to breaking very easily, which is what ollie broke. they operated to insert pins to hold that small bone in place. Ollie was super hard to sedate or keep still, he actually bent one of the pins, and took a while to heal up b/c he was so active on his broken leg, dork.
He still gets water on the knee b/c of that injury, the pins rub a little on the skin causing inflammation and water building up (jumping is the worst for it). they would have to put him under to take the pins out, which isn't really worth it. I really limit his jumping even now at age 3.
don't mess around, get an xray, it's the only way you'll know for sure what's going on.