Now take this not as advice, but just as my experience. About 1 month ago my very active 6 year old jumped off his bed, and was not limping, but we knew he jumped very hard because I heard him land, on his feet, from the other room. The day after he started limping, but we couldn't find anything, brusing or swelling, he limped for 2 days and I finally took him to his ped. He checked him and said there was nothing wrong, that he didn't see the need for an x-ray, it was either that he put too much weight and bruised a joint internally or was just sore after too much activity. He did recommend getting bloodwork, since sometimes kids might have some kind of arthritis or something, but everything came back fine. He was doing just great a few days later, but we have noticed that if he jumps a lot, or from high places he'll limp for a few hours, so I think it's putting too much strain on the bone/joint. Long story short, he was fine, but we had to pay $30 for dr. visit, and over $100 for bloodwork. Now this is advice, watch him over the next few days, and if it gets worse or doesn't improve in a at least 3 or 4 days, take him to a doctor.