What can be the cause of vertigo in a 30 year old woman pilot in steep dives?

Inner ear irritation. It is possible that the steep dive leads to negative pressure in your middle ear which in turn is causing irritation in your inner ear. The inner ear has a membrane called the round window which can be irritated by rapid pressure change leading to vertigo. You should see an ear specialist for a further evaluation.
Illusion. During rapid acceleration such as during a steep dive or if there a sudden change in airplane attitude, the balancing system of the inner ear sends a signal to the brain. This signal does not match the signal that the eyes are sending to the brain. Vertigo occurs.