What could cause a womans scalp to tender in the crown region only during her menstrual cycle?

Hormonal change: During your cycle there is change in hormones. Your hormone system nervous system immune system are interconnected. It's because of this that you can have these symptoms.

Muscle tension: Muscle tension and myofascial pain will refer to the crown of the head. Hormonal changes of the menstrual cycle may be leading to increased muscle tension.