I'm 23 years old and have recently started playing soccer for a club again after a 3 year absence (although I have often been playing with mates and during work lunch breaks and have been playing tennis on a fairly regular basis). Anyway during my second training session I felt a sharp pain behind my knee. I immediatly stopped playing and began the notice the same pain behind my thigh. At first I thought it was a simple hamstring strain and thought nothing of it. Its been over a week since I first felt the injury and I have started to notice that my knee is feeling unstable and weak and the pains behind the knee and behind the thigh still persist, but only when I do hard physical activity, I can jog with no problems. I am exteremely worried with the unstableness and weakness of the knee because I had a knee reconstruction (a full tear of the ACL) when I was 17 on the same knee. Is it possible to have a weak knee from a hamstring strain or have I done something more serious. I don't think I have torn the ACL because when I tore it the first time when I was 17 it was extremely painful and I couldn't walk for 2 weeks, which is different from the systems this time round.

Any help would be appreciated.

Thank you in advance