This has been happening for a couple days now. The first time, my hamstring on my left leg, above and below the back of my knee, got this odd pain and totally gave out, and I fell. Its happened atleast 5 times a day since that, and now I dont fall because I can feel it coming, and put my weight on my right. But since Im in sports, this is a serious issue! Any ideas on what it is, or what I should do?