Both the instead cup and tampons sit quite near the cervix and if they are coming in contact with your's it could be what is causing the cramping. A lot of women have a tempermental cervix.

The Keeper, Diva cup, and Mooncup rest in the vag in a different way and are much less likely to come in contact with your cervix. I'd say it's still a good shot, especially since you don't seem to have a problem dealing with the ick factor of cups, which is what a lot of women balk at.