Welcome to Apug!
This artefact indicates unintentional exposure from the backside of the film.
(As the red-sensitive layer, which is the lowest, next to the film-base, is the same time the most sensitive one. The red couplers and a red filter on top of this layer shield the layers above.)
Look for a light-leak in the back of your camera. Or at the side or front if light from there could reach just the film on the take-up spool, if it is wound base-side out.