It might not be the lettuce itself. A lot of restaurants put a perservative on the lettuce to keep it fresh and crisp. I simply can't eat a salad at The Olive Garden chain eateries for that very reason. It could also be an additive that most restaurants put on their food, such as MSG. Even fast food places use it.
A simple test will determine if the lettuce is your problem. Make a fresh salad at home, from fresh head lettuce. Throw away the outer leaves in case the grocery has sprayed them with an additive. Don't use a pre cut, pre bagged mixture of lettuce. It too could have additives. Use other fresh vegetables for your salad. Just once, eat this one without dressing, which could also contain additives.
If you don't have a IBS reaction from this fresh salad, you'll know it's not the lettuce. If you do, well then you'll know it is the salad.