Where was you op when you overheard them talking about you? I would have been tempted to fly in the room shouting ''hellooo!! i can friggin hear you!!''
Talk to them as everything you do that isn't agreeable will be put down and dismissed as having MH problems 
I have flare up of IBS. Most people think its a dicky tummy or prone to having the runs. Nothing to do with having vice like pains, sweats, feeling like you are about to pass out, the runs with any type of stress..excitement,anger,upset. Clinging on to the toilet as you feel like your insides are about to fall out....
I scared to death my manager one day, he was in a meeting with a client and I was having a flare up. The pain was so bad and i was sweating i really thought i was going to pass out and no one was around. So i crashed in to his office and crawled on to the floor! Both manager and client slack jawed.
Im not surprised you dont like ad hock outings, there is no fun thinking your going to shit your self in a bloody car on the motorway or pushing a trolly round asda.
Print of some info pamphlets, give them to read. They are all being out of order.