I feel severe pain in my belly while masturbating?

Possible: sometimes the nerve is irritated referring up into the abdomen there could be a hernia referring into your abdomen sometimes a low grade prostate infection can cause it go to your provider for some tests

Lubrication & Doctor: Masturbation should not be a painful process and the fact that you discribe your episodes as "severely painful" would suggest to me that it should be evaluation by your doctor. Trauma to the local area while maturbating could explain your symptoms and might be relieved with a water based lubricant.