Steph,
If you had only one or two marks with that shape on a whole roll, then the kinking theory might hold up, but to have it on every frame, in the same place and same shape would suggest to me that it is a light leak. One other thing to try would be to shoot four or five frames in rapid succession with the camera inside your changing bag, and wind on again - if you don't get the shape on these frames, it's definitely a light leak.
Just a thought