Restaurants are VERY tricky. Even if there was nothing in your salad that contained wheat you might have gotten cross contamination from something else. You might want to look into the Triumph Dining cards, or something similar. They describe to the chef/wait-staff what you have to avoid in terms they can understand. It really makes eating out a lot easier. Usually they take the card back to the cooks then come out and say what, if anything, I can order off their menu. But without being able to check the ingredient list yourself, you can't 100% rule out that you're not getting wheat.

Could your chocolate mints have been sweetened using something other than sugar? Some artificial sweeteners will give most people terrible gas and/or diarrhea.