What Causes Recurring Pain On Back Of Knee?

About a year ago, I experienced a sudden pain at the back of my knee, while doing nothing in particular, and didn't think any more of it until I noticed a large very dark bruise in the same area.
>A few minutes ago, I just experienced the exact same pain in the same place at the back of the knee. Too soon to see a bruise, but assuming one forms, should I be concerned? Should I seek medical treatment?