A swelling of a single leg is usually more ominous than swelling of both legs.
It can happen because of
1. Blood clot in deep veins of the leg
2. Varicose veins of that leg
3. Filariasis
4. Cellulitis ( soft tissue infection )
Low
thyroid levels end
kidney problems will cause swelling of both legs
It all depends on the presence of other associated symptoms like pitting in the swelling, pain, redness, etc.
Next Steps
Seek medical help at the earliest
Health Tips
If you have had a long bus/flight journey just before the onset of swelling, you must consult a doctor immediately and rule out blood clots on the leg by scans, bcoz they are otherwise life threatening