Welcome to the forum! Unilateral (one sided) leg swelling could be due to trauma to ankle joint, ligament or muscle sprain. However all these will cause pain too. Venous pooling as in deep vein clots, thrombophlebitis, and DVT (deep vein thrombosis) is another possibility. Pelvic tumors and bone tumors too can cause one side swelling. Infections like filariasis and cellulitis are other possibilities.
Hope this helps. It is difficult to comment beyond this at this stage. Consult your PCP to start with. A comprehensive investigation is required keeping all the points in mind. Please let me know if there is any thing else and do keep me posted. Take care!
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