Unfortunately most people don't understand what they can't see or at least don't try to and simply judge you on how you look on the outside. You look fine, even if you're slumped in your chair holding your abdomen groaning in pain. I look at it as two different types of people. The people who care and will listen to you, help you and understand you by at least looking up your condition and there are those people who would simply prefer to stay uneducated. From my experience, anyone who doesn't bother to educate themselves (by at least listening to you and accepting your word) isn't worth the stress of being around. Unfortunately sometimes these people are our bosses, teachers, family members, basically people you can't get away from. Honestly I only talk to them about my condition when its really needed because the stress they cause isn't worth it and can actually make you sicker.
What some people have done is carried around a picture of their colonoscopy and shown people so they would back away and quit hassling them. Its not a bad idea but I'm sure we all wish that people would just take our word rather than treat us like crazy liars out for attention. I haven't tried the picture method myself but I have gotten rid of friends and a boyfriend of five years because they weren't worth the stress. Only people in my life now that judge and don't understand still are family members (yes after 21 years there are still family members who simply haven't learned, they just don't want to).
Eventually you will find your own coping method and teaching method and learn who is worth your time.