Sorry to hear that you are having pain . I also think you might be rolling a little bit your belly and thats why it may be hurting (in your stomach) . Just to be clear on the difference here are 2 videos showing the difference :

Sahira's undulations , I find her very clear and defined ,
Egyptian Belly Dancing : Egyptian Belly Dancing : Undulation - YouTube

And this is a small video I did as part of a project to give revision lessons to b.d students . This is the belly roll, sorry for my weird accent , my students

How to do a belly roll / belly dance wave - YouTube

--- either way , you should speak to your doctor , you should be able to dance freely without having any pain xxx