The circled area on your picture is the Peroneal Tendon. It is common to develop tendonitis on this tendon from simple overuse. Tendonitis is when the tendon becomes inflamed and irritated. Commonly this is caused by repetitive motion and continuous overuse. The best thing to be doing for this is focus on cold therapy. Put a cold compress on it a few times a day but most importantly right after being on it. The cold will bring down the swelling and inflammation and will help ease the pain. Since Tendonitis causes your tendon to become inflamed, blood flow to the area also becomes impaired. After focusing on cold for at least 3-4 days, start treating with anything that increases the blood flow to the area (massage therapy, ultrasound, BFST, acupuncture, etc.). These types of treatment give you the nutrients and oxygen you need to heal but reduce the risk factors involved with a lot of physical activity.

Of course an MRI is always the best thing to do. It would be good to determine no tearing is present.