I do not think you are using good lubrication for masturbating and this is casuing you problems of injury
to those areas. The pain to your abdomen is reffered pain. Meaning that it is not exactly at you abdomen but you feel that the pain is present in your abdomen. What you can do is to use a lubricant like moisturiser, or soapy water next time while masturbating. This will help you greatly.