Weird bruise thing behind knee

[Dianyla]

So I was trying on some bike shorts this afternoon and was looking at myself in the dressing room mirror and I noticed a purplish dot on the back of my knee. It's the color of a bad bruise, and fairly small. It feels a little twingy but not painful. The skin over it looks good, it's definitely subcutaneous showing through. It's in the "dimple" at the back of the knee joint.

It's in a hard to see spot without a mirror so it might have shown up days ago for all that I know. I did so some harder than usual riding a few days ago, could that have caused it?

Any ideas? Is this a varicose vein?

Attached pic (kindly ignore the stretch marks and the shaving rash, please!)