It could as simple as a sprain or it could be the knee popping in and out of place, also called Luxating Patella. My female yorkie (1 1/2 yrs. old) experienced the same limping off and on symptoms, unfortunately, she has a deformed hip also referred to as hip dyplasia. It doesn't slow her down any though and I am treating her. I hope for you and your baby it's just a minor sprain.
They are wild little ones and fearless when it comes to jumping.