Running at 4 months

I’m feeling really bad. I took my 4 month boy to the park two days in a row.. we were running together and he was chasing me. He had a weird limp a couple hours later in his back legs. Im really worried and was not super educated that he shouldnt be running much because of his bones and joints. Should I keep an eye on him and see how hes doing? Or take him straight to the vet? He doesnt seem to be in pain, hes still very playful and wants to run in the house. I’m just very worried