Swelling of any part of the body is usually due to fluid retention at that region.
There are 3 main causes for swelling.
1. Heart related
2. Liver related
3. Kidney related.
Sometimes local swelling may be due to injury to that part.
Swelling may also be due to obstruction in the flow of blood.
In your case, the feet swelling may be the result of obstruction, as you are saying that it is more in one leg.
Want to know since when you are having this problem
Any particular time of the day?
Any particular position, like sitting, standing or traveling increases the swelling??
Have you checked your BP?
How is your urination, frequency etc?
Reduce intake of salt in your food.
Keep your feet above your chest level while lying down.
You can consult directly for treatment