Peroneal Tendon Injury

Definitely sounds like peroneal. I know you probably don't want to hear this, but you're probably better off not running until you can run pain-free. Whether it's tendonitis or a tear, this is probably your best bet. I went to physical therapy for mine...was also forced to do 42% of my IM run training in a deep-water pool because of it. Take care of it before it becomes more serious. It seems like it can become serious rather quickly.

Mine came on in the oddest way. I went for a 60' run in the morning and everything felt fine during the run and after the run. Then, at the end of the work day (like 10 hrs later), I stood up at my desk, got a shooting pain in my foot, and that was the end of it for me. I did try a run on it after that, but that was a bad idea. My docs still don't understand how I was able to run completely pain-free that AM. So, just be careful. Best of luck to you.