In the order of dangerousness: first, venous doppler (ultrasound of the leg veins) needs to be done. Most important is to rule out blood clot. Second, this test should also be geared toward checking the condition of your deep veins, to make sure you don't have venous insufficiency and venous reflux. If there is no blood clot there, compression stockings, knee-high, that should be put on immediately upon awakening, BEFORE you put your feet down, may help.
Another possible cause could be swelling caused by amlodipine which you are taking for your blood pressure. I would, however, expect it to be on both sides. HCTZ is a water pill, used for reducing the edema and therapy of blood pressure. It can cause electrolyte abnormalities, such as low Potassium and low Chloride, which may contribute to edema. HCTZ also causes high Uric Acid, which can cause gout, but it usually goes with pain. If all is normal, and your blood pressure is not well controlled, maybe asking your doctor to increase your HCTZ, so you can treat both the blood pressure and swelling. I hope this helps. Good luck.

Petar N. Novakovic