The Keeper and the Diva cup both have return policies for just this reason (three month and one year respectively). Does the tampon/Instead cramping happen more at any given point in your period? I know I can't wear a tampon or now my Keeper for the first day and half but am okay after that. Also, have you tried cloth pads? They are much more comfortable and don't make you feel like you have a wet garbage bag between your legs.

Or what about a sponge? They also absorb but are much softer than any other internal product, if the cramping does affect your vagina you might have to wear a pad in case your vag wrings the sponge out, but it should be better than nothing.