I've got a VERY low cervix during my period. The first cup I got was a Diva(size 1). The problem that I had with it was that it would sit the cervix inside of it and then push, at times painfully, especially when I tried to do the 360 turn, way further beyond the cervix on all sides.
So I though smaller, right? Bought a little lunnette. In theory it should work, but whenever I use it I get HORRIBLE cramping. And a full, crummy, drank-too-much feeling in my lower belly. And I get that even on dry runs.
I'm about to give up here. The only thing I can think of about the cramping is maybe it's blocking the cervix like a cork rather than leaving space there.
When I use them on my period, they don't even get a quarter full either. I think my period just trickles. I switch back to pads and two days later my period seems back to normal.
Maybe there's a slight stress factor? I got the first about four months ago, but I don't really use either anymore.