When my dog was about 8 months old we were at a friend's house. They have 7 fenced acres and she was flying everywhere. She had a blast. After a while she came in and went to sleep. When she woke up she was limping on her rear leg. At first it wasn't too bad, but it got to where she was really gimpy. We looked several times for the cause, but had a hard time finding it. After a couple of hours we were able to see she had a very swollen toe.
She kept wanting to play the whole time, just hopping instead of running. It was almost impossible to keep her calm. The next morniing wasn't much better so off to the vet we went. It turned out she broke her toe running. The vet said it happens all the time and there wasn't too much he could do. He offered a cast but said it wouldn't make much difference in the end.
We didn't get the cast and in a couple of days she was much better, with all limping gone within about 10 days. It never did affect her attitude though.
You never know, keep looking to see if you can fnd the cause, I'm sure your dog is fine, but if it doesn't go away and you can't see the problem, sounds like a trip to the vet is in order.