Upset stomach she experiences after drinking milk is caused by too little lactase, an enzyme that breaks down a sugar in milk into monosaccharides. which monosaccharides form from this reaction?

The monosaccharides form from the reaction are ONE GLUCOSE AND ONE GALACTOSE MOLECULE.
Lactase is an enzyme that is located in the small intestines of humans. It helps in the digestion of whole milk and dairy products by breaking down lactose into glucose and galactose.  The glucose produced in the reaction will be absorbed by the small intestines to be used for energy production in the body.