You don't have to go into graphic detail unless he absolutely needs to hear it. I told my girlfriend that I have a chronic digestive disease and IBS.
Tell him to Google "IBS," look into it and see if he's interested in hearing more. My guess is that he'll have read more than enough by that point and will understand where you're coming from. You can also just say "my intestines are inflamed, and it's chronic."
Something as simple as saying "I have a bad, sick pain in my gut right now which doesn't make me want to have sex" ought to get the job done, no?
I'm sure there are plenty of times where you DO feel up for it, so don't worry about
withholding. It's easier for girls to get away with that anyway