It could have started from a tiny little scratch. I haven't been keeping up with cacti knowledge so my advice is probably not the best atm.. But sounds like some kind or rot or fungal thing. I can only recommend cutting the tip off until you get to a non squishy part... Then dust the top with some sulfer to stop the rot getting back in.. uhh I'm sure someone else should chime in... Also some better pictures could help us figure out what's wrong with her...