side effects of topical corticosteroid overuse include thinning of the skin, permanent stretch marks in body fold sites such as the arm pits and behind the knees, swelling of tiny blood vessels beneath the skin surface especially on the face, hypopigmentation and slowed wound healing.
ref:http://www.aad.org/media/background/press/corticosteroids.html
You will have to stop using the drug and wait and watch . In most cases once the drug has been stopped the skin usually regains the pigment.