My 2 mo old has very dry scalp, and she is always scratching it so she looks like she has dandruff. What could it be? Her dr. Says nothing...

Cradlecap. Initially you may try using baby oil and let it soak for a couple of hours on your babies head, and then scrub it off. You may also use head and shoulders or selsum blue, 3 x a week to see if helps (just dont get in eyes), if not helping a prescription of Nizoral (ketoconazole) shampoo, 2-3x a week for a month and then periodically.
"Cradle Cap" This is a combination of retained body oils & dead skin flakes sticking to the scalp.Baby shampoos, in an effort to be tear free, don't contain the chemicals needed to separate the flakes from the oil. Moms are sometimes reluctant to scrub the area based on fear the "soft "spot might be hurt. Head &shoulders/ tegrin/selsun+ soft cuticle brush can make short work of this problem.
Cradle cap=seborrhea. Cradle cap = dandruff. Normal at her age; but can rub in vegetable oil with a soft toothbrush. Or rub scalp with dandruff shampoo over 10minutes, then rinse well over a sink (so the selsun doesn't rub over the rest of her skin).