Your 12 week old needs the correct food: grower. Incorrect food and the addition of treats can cause her to have more than just diarrhea, yes.
Laying feed is only for birds over 5 plus months of age.
Iceburg lettuce is a water-filled useless food for birds. It's alright as a treat, but will cause diarrhea and - in large amounts - can cause the diet to be insufficient. Using a stronger green is a better treat. Collars, mustard greens, pretty much greens you'd cook.
You can also try giving her a tiny bit of plain unflavored yogurt. It has live bacteria in it (that's the benefit, not the milk portion). The chicken's digestive system is reliant on live bacteria to digest their food and help reduce the incidences of diarrhea and digestive upsets. So replacing the live bacteria with more is helpful.