What ligament or tendon is this?

cyork10

I know this is really vague, but I'm going to do my best to describe this. Last Thursday I got up out of a chair and there was no twisting involved. I suddenly had a really sharp pain in my left knee that caused me to have to sit back down immediately. There was no popping noise either. It was difficult for me to walk for the rest of the day. It started to feel a little better the next day and through the weekend, although it still felt a little unstable. Monday it started to feel better and I ended up going to the doctor on Tuesday for a sinus infection. While I was there I asked about my knee.

The doctor applied pressure and moved my knee in different directions and determined it was probably just fluid back up or something in my knee that caused my pain and that I could start working out again after I was over the sinus infection. Since then I have occasionally felt a pain in an area by my knee when walking. Before I determine if I should go back in I want to do some research on what exactly this pain could be. The area that is hurting is on the inner side of my left knee. I say knee because that's the best way I can describe it. It's actually like a half inch below my knee on the inner side. I can feel a slight pain when I apply pressure to the area. Does anyone know what ligament or tendon could be causing this pain? I really appreciate any insight provided! Thanks in advance!