I Am Experiencing Pain In The Back Of My Leg

I am experiencing pain in the back of my leg (approximately 12" divided equally up and down at the knee level.) It feels as though it's deep inside, like in a vein. Walking or moving it does not improve the pain. I am suspecting a blood clot in the vein. It is not bulging nor is it hot to the touch. I've had it before, although eventually it goes away. What are your thoughts? I do use an eliptical machine on a regular basis. Could that be relevant to this issue?