I have been running for years, for fun as well as competitively. For some reason lately the beck of my knee has been hurting me really badly. I have been off of it for a month, doing more low impact things such as biking and swimming. I had been running 5 days a week about 5 miles a day. This is not an issue for me whatsoever. I didn't feel myself pull anything. Do any other runners have any advice on stretching or what this might be?