Haven't pinned down the cause. In June I ran 30, 30, 40, 40 mile weeks. Most of that was easy paced. But that's exactly the number of miles I ran in May... Could be old shoes, which I have now retired. Could be new speed workouts with track club. Probably all of the above.
Nothing serious though. I'm running a 15K tomorrow (with Tylenol)!
Two little runners