I can totally relate to the friend problem! I am relieved to know that I am not the only one! I had a huge issue last year when I lived in my sorority house. My so-called "best" friend at school was convinced I was anorexic because I lost so much weight and I read labels all the time and I watched everything I ate. We all know why did that, but she was sure I was anorexic. Instead of learning about IBS which she knew I had, she proceeded to talk to all of my sorority sisters asking them if they thought I was anorexic. Pretty soon the entire house was believing I was anorexic because of this "friend."
Then she decided that the best thing for me was to confront me and tell me everyone thought I was anorexic. When she confronted me she TOLD me I was anorexic. When I tried to explain everything to her she said I was in denial and that I used IBS as an excuse not to eat. She said another friend of ours had IBS and she could eat whatever she wanted. I told her that everyones IBS was different, and that this friend had just been diagnosed so her habits may change (and I question if this girl even has IBS).
Needless to say, I couldn't convince her and we got in a huge fight. I decided to put a stop to the rumors and I called a house meeting and I stood up in front of 32 girls and explained EVERYTHING! YIKES! I told them if they really were concerned about me to come and talk to me and I would GLADLY explain IBS to them to help them understand my condition.
Luckily, I was able to convince most of the people (the girls I consider my REAL friends). The OLD "best" friend and about 3 other girls and I will never resolve this issue, but what can you do? Obviously they were not true friends. It was a difficult realization to come to!
Just hang in there, it sounds like your friends really care about you and if they are true friends they'll accept your condition and want to learn more about it!