(for 14 yr old daughter) Is it normal that she has diarrhea with undigested food like (lettuce) to happen during the last days of her period? (Day 5/6

Not typical. In 40 years of pediatric practice, one one has brought this to my attention, so I believe it is unique for your daughter. It is true that dietary changes and hormones surrounding menses can cause a change in bowel movements, and any food that has a high fiber content may appear undigested in stools. If she feels well, I see no need for concern.