Menstrual cup back flow and worried. Super heavy flows.

I have super heavy flows and have used almost every cup. My goldilocks cup seems to be my Nixit Cup. I usually can tell when Its full cause I start to cramp a bit. I am worried about backflow and overflowing my cup and possibly spreading my old tissue over my organs and possibly causing endometriosis. I have had a miserable messy period this cycle and the Nixit is the only thing that helps. I changed it and cleaned it with soap before bed and my flow was SO heavy and uncomfortable that it woke me up in the middle of the night and it was packed. Has anyone else face these issue or fears?