Before you shoot a frame, pop the back in bright sunshine, and mark the middle of the film with a Sharpie. Put a large V for vertical and H for horizontal. Then replace the back, take your shot, and advance the film.

I suspect you have stored your camera on its side and the glass has flowed over time to one side of the lens mount to the other, making vertical and horizontal difficult to identify...

Isn't this question 7 months late, John?