Brief Answer:
Injury to tendons, ligaments, bones of foot/ systemic diseases can cause it

Detailed Answer:
Hi XXXX,
Thanks for the query on Health care magic platform.
I understand tour concern.
The swelling on left foot can be with pain or without pain.
1. Swelling without pain- It can follow over use of the part ( long standing with weight on left side )/ severe anemia with hypoproteinemia/Infection like filariasis/
systemic diseases involving heart, kidney, liver, thyroid.
2. Swelling with pain can follow -
- Stress fracture of one of small bone of foot,( due to osteoporosis in advanced age,
-Ankle sprain.. This causes injury to lateral ligament of ankle joint,
- Cuboid syndrome-Ankle sprain if severe can cause dislocation of the Cuboid.. a small bone in foot,
-Inflammation of tendons on outer side of foot - this follows long time over use ( jogging, running) or wrong position of foot creating imbalance of muscles.
-Corns & calluses/ injury to the outer side of foot can get infected can result in inflammation swelling & cellulitis of the part- associated with fever
- Arthritis of ankle joint can cause swelling on outer side of foot. This swelling usually comes & goes.. making movements painful
* A physician should be consulted for examination,needful investigations to find the cause , followed by specific treatment . He may refer you to concerned specialist if needful.
I hope the answer is both useful & informative to you. Any follow up query is welcome.
I request to grade the answer( if you are satisfied with it ) & close the query . Thank you.

---