mummylion I had a similar thing a couple of weeks ago. I'd run a HM without much training on the Monday, then spent a long time in the car motorway driving on the Weds (with foot in the same position for many hours). By the middle of Weds night I was in agony - tried to stand up to get painkillers and passed out with the pain.
My xray was clear, but apparently that is quite common. I have RICE'd for a couple of weeks and am taking it easy, not running much and the pan has gone apart form an occasional twinge. My problem stems from an injury on my right side (tendon/calf) which has affected my gait and put more pressure on my left side (where the possible SF was).
I hope you feel better soon.