These last two weeks I've resumed running, having had. 4 month break. Typical daily run is 3 miles. Last Sunday, went out with a friend and ran around 5 miles. Result was a swelling behind the knee. It's soft and seems filled with (synovial?) fluid. Knee joint or swelling isn't painful at all. Selling feels a little tight but that s all really. So the question is - what is it? And can I carry on running. Really don t want to stop as enjoying getting back to it on a daily basis