I'm Having Some Trouble With A Ligament In My Knee.

I'm having some trouble with a ligament in my knee. It's on the inner lower side of the kneecap and you can feel it sliding back and forth with your hand when I lightly swing my leg freely while sitting on a bench with my feet not touching the ground. It is painful if I try (for example) to exercise on a leg extension machine, where you're seated and lifting the bar with your lower shins. It's painful going down stairs or in warrior stance in yoga. Several times a week (beginning several months ago) my knee will not fully straighten immediately, and then as I walk around or gently persist in trying to straighten it, with an audible pop it will suddenly go into place and feel fine--and this is without pain at all. Four months ago, I was pedaling a stationary recumbent exercise bike and suddenly felt as if a small hammer had rapped me on the knee in the location where I'm now having trouble. I looked at my knee to see what had hit me, but of course it was only internal. There was a small amount of pain, so I stopped exercising and went home and put an ice pack on it, took ibuprofen, and rested; the next day, my knee seemed ok. Prior to that, this area of my knee did suffer some trauma when I banged it on a ridge of the steering column in the car, which was very painful; but it did not seem to be injured after that. Once a couple of months ago, during cold weather, for about one day I could not fully straighten my knee at all because of a painful swelling sensation inside my knee. I was able to walk around without too much discomfort anyway, but could not straighten my leg fully until about 24 hours later. This last symptom only occurred once. What do you think is wrong? Do you think I need surgery? Thank you!