I Have An Ache In The Back Of My Knee,

I have an ache in the back of my knee, it radiates up my thigh a little and down my calf a little. it isn't a real bad pain but it doesn't feel like a muscle strain and if anything I have sat more than usual. I noticed it when I drove for two hours today and felt like maybe the seat was maybe touching my leg in the wrong spot. could it be a blood clot? I am 59, smoker and don't really exercise much. take BP and cholesterol meds