What causes one leg swelling?

Several things: It could be cellulitis, deep or superficial venous thrombosis , lymphadema.

Many things: Leg vein disease. A blockage in the vein drainage in the pelvis also can cause this. This can be the result of a mass in the pelvis such as ovarian cancer. A blockage in the lymphatic system can also cause this. The lymphatic system drains lymph which is the clearish fluid that you see in blisters. Have a Dr. evaluation.