Swelling and pain on one side of the foot may be caused by acute injury, arthritis, varicose veins of the lower extremities and other factors. 1. Acute injury: for example, acute sprain or heavy object injury, so that the soft tissue of the foot is damaged or fracture, it will cause pain and swelling of the foot, may also be accompanied by subcutaneous bruising, restricted movement and other symptoms. 2. Arthritis: For example, osteoarthritis, gouty arthritis, rheumatoid arthritis, etc., after the aggravation of the lesion, it will lead to swelling and pain on one side of the foot, which may be accompanied by morning stiffness, and in serious cases, it may lead to joint dysfunction and deformity of the joints. 3. Varicose veins of the lower limbs: it may be related to venous valve dysfunction or venous sclerosis, etc. In the initial stage, it will cause the lower limbs to have symptoms such as soreness, heaviness, and fatigue, etc. With the aggravation of the lesion, it will lead to pain and swelling of the feet, which may be accompanied by numbness and itching. If the swelling and pain on one side of the foot persists without relief, it indicates the existence of an abnormal situation, and it is recommended to consult a doctor as soon as possible for examination to clarify the cause of the disease.