Huh? He said that an initial ultrasound was inconclusive. I'm curious what aspects of his description led you to to know it was a blood clot. He didn't have a surgery, so that lessens the probability of a blood clot.
Based on what information he provided, you should be able to think of several different ailments/conditions/diseases immediately. I want to know what those other possibilities were and what led you to say --that is a blood clot.
What gave it away quickly to me was the coolness of the ankle. But in Nursing school we're taught the "5 P's" - Pain, Pallor, Pulselessness, Paralysis, and Paraesthesia.
He had pain upon movement, blood clots obstruct, causing pain. When you stand up, your blood rushes to your feet, so the discoloration upon standing indicates that circulation is obstructed and blood is not getting properly circulated throughout the body (rush of blood -> blockage causes his pain). Cool to the touch is the sign we always look for in post-op patients who are at risk for clotting. We also check out peripheral pulses, though the OP probably did not know to do that. A quick check of the pulse on the top of his foot or close to his ankle could indicate a lack of blood flow.
At the same time, the OP described the "pulling" pain between his calf and ankle occurs when he is walking. When we do checks for clots in the hospital, especially on the postpartum female, we'll check the patient's "homan's sign," by slightly pushing their ankle forward while their knee is fully extended (as you would if you were walking). The patients that are positive will describe the feeling as "resistance" or "pressure," but pulling pain seems to point in the same direction.
In any case, blood clots occur quite often, and not just in post-op patients. You can get them due to age, medications (especially birth control, not that OP has a problem I'm sure), broken bones, immobility or sitting for a prolonged period of time (a la airplanes), family history, heart disease, and more. Sure having recent surgery puts you at a greater risk, but it's not uncommon to get one otherwise.
Hope that helped a bit.