For some people Lettuce is hard to digest.
Some people have a harder time of digesting lettuce and other similar vegetables.
The Lettuce is still healthy for you and you should eat Lettuce at least once in awhile to get the nutritious minerals, vitamins and other health benefits of lettuce.
If your stomach is hurting after eating lettuce it could be you have digestive issues and may need to avoid lettuce or at least not eat as much lettuce.
Lettuce can be hard for some people to digest because it's fibrous.
If you're digestive system is not working properly or you have an unhealthy gastrointestinal tract or food sensitivities, then you're more likely to have a bad reaction to digesting raw vegetables including lettuce.