My stomach hurts every I wear a menstrual cup. Why?

The pain is most likely caused by the fact that the cup touches your cervix. It is not dangerous but it can feel very uncomfortable or even hurt.

You can avoid this by placing the cup as low as possible. If the cup rises by itself try this little trick to avoid it: when you insert the cup let it open before it is completely inside your vagina, and push it in when it has opened. This way the seal won't become so strong and the cup shouldn't rise so high.