Menstrual cups...

After reading some of you describe how much y'all loved the diva cup I purchased some soft cups at my local wal-mart....and while inserting them was a little scary everything went fine, no leaking at all but I had severe cramping while the cup was in. Am I doing something wrong? Is this normal? I would really like to use these because I think they are wonderful but I already deal with cramping and don't know if I can handle them being worse due to the cup. Any information would be greatly appreciated!!