How Can Knee, Calf And Ankles Pain Be Treated?

Hi, I started getting severe pain on the inside leg part of my knee. I am in extreme pain even when lying down but especially when bending leg to i.e. climb stairs. The pain is also going down my calf and to my ankles. I end up limping quite a lot. then for a few minutes I seem to be fine, and then it starts again, Not sure what could be causing it. Help please!!