Nixit Problems?

So I used my divacup faithfully for 10 years, but it was time to retire old faithful and get a new one. I'd used disposable menstrual discs a few times when I had left my cup at home and needed something quick, and I loved them, so I decided to try Nixit, the reusable menstrual disc.

Anyways, I keep dislodging it. If I pass gas or use the bathroom, I manage to knock it out of position and leak everywhere if I don't fix it. I'm not coming close to filling it, I just apparently move too much down there. I'm scared to trust it too much. What if I cough or sneeze and it comes loose?!

Is this user error or is the Nixit just a bad fit for me?