Pain in the side of the thigh may be caused by trauma, tendonitis, and strenuous exercise. 1. Trauma: It may be caused by collision or blow to the side of the thigh. Due to localized injury, thus pain can occur. 2. Tendonitis: This is an inflammation that occurs when a tendon attaches to a bone or the beginning of a muscle. It may be caused by long-term movement of the muscles on the side of the thigh, repeated strong pulling, and so on. Due to the stimulation of inflammation, patients may experience pain. 3. Strenuous exercise: it may be caused by the patient running for too long, jumping exercise, etc., the accumulation of lactic acid on the side of the thigh, thus the pain appears. Pain in the side of the thigh may be caused by other reasons, it is recommended that the patient go to the hospital in time and follow the doctor’s instructions for treatment to avoid delaying the condition.