Yes. You need to stop running immediately or your cock is going to crawl up inside your body and stay there. But seriously, it's normal. Your body is made mostly of water, so there will be fluctuations in size among all your tissues. Not only that, but there are muscles that cause your equipment to contract when it is threatened physically. Everything just tightens up down there from the impact of running. It happens to me, and to most guys. I come back from my run and even on warm days, I get some shrinkage. Don't be worried.