Ligament or Tendon Behind Knee Pain

Hello All, a few weeks ago I noticed some pain behind my knee. The part of the knee I am speaking of seems to be the ACL but I'm not 100% sure. When I completely straighten my leg and point my toes up, it's the tissue that you can feel when you put your hand on the back of your leg behind your knee. I first felt the pain when putting my feet up and crossing my ankles. It felt as if I might have very slightly hyper extended my knee. The problem is that the pain has never subsided and it's gotten worse.

Here is the wierd thing... until today I have been able to run, jump, jog, or do any other activity with little to no pain at all. I play and coach basketball 4-6 times a week so I am fairly active and this has never been an issue. When I would feel the pain would be if I put my legs up and crossed my ankles (much like when I first felt the pain). When it would hurt the absolute worst is straightening the leg after I sat with my legs completely bent at the knee (indian style or like a little kid sitting on his legs). That would really hurt, but only if were straightening from a it being fully bent for a decent amount of time.

Today was the first day (playing basketball), that I had pain. I was knocked down by another player and landed completely on my side. I can say that normally this type of fall wouldn't hurt at all, my right leg landed completely on it's side from the hip to the knee with no extra pressure on it. Usually no big deal, but this one really hurt that same part of my knee. Now it seems to be an ACL issue, but the fact that it doesn't hurt during normal activity has me stumped.

Any ideas??

Thanks so much!