I can certainly prove I did a whole bunch of work for a fairly lengthy period, so could be something worth me exploring. Thanks for that! Was about to just write it off
Get a lawyer and good luck!
But yes, the other user was absolutely right: just because a contract isn't written, doesn't mean it's not a contract.
I suspect they're not happy with something (this happened once before I was full-time, I knocked the money off for a single task that they weren't happy with, which was something I know I'm not good at, but tried anyway). But like... you can't play those games with someone's full time salary. It's insane to think that's okay.