I think Ed's got the answer, I remember having a similar problem in my early days caused by my novice fumbling loading a film in a darkroom leaking light and this gave me a partial positive image. The problem was compounded as I had been told by a member of the camera club that I had joined to dunk the film in a weak solution of hydrogen peroxide to help it clear and speed up the process. This caused dichroic fog suggested by Jorge as a possible problem. I cleared the dichroic fog by mixing a weak solution of Thoiurea and Citric acid and soaked the negatives in it. It's a very dodgy process for it reduces the emulsion to a jelly and if left too long in the solution the emulsion is removed from the backing material. I remeber trying this with an unwanted negative on the strip before I cleared the important negative.