What caused my foot to swelli?

Swelling: This is not enough information. Was there an injury? Is there a history of inflammation, arthritis? If not, i would look into some type of venous dysfunction as a cause. See a podiatrist.

Many possibilities: Some causes of swelling of the feet and legs can include systemic problems such as hypertension, and problems with the kidneys and localized issues such as arthritis, infection, lymphatic obstruction, blood clots, varicose veins, trauma (fracture, sprain, tendonitis) and side effects to medication. Have it evaluated by a professional and get the appropriate treatment.