Because the scalp is accustomed to constantly being stripped of all of its precious sebaceous oil through chemical shampoos, it over compensates to protect itself and keep itself healthy by over-producing this oil. Basically, your scalp goes into shock every time you shampoo with sulfates and though you’re not washing with them anymore, it takes time for your scalp to catch up to what is happening. This may happen for as long as two months, until your scalp has a chance to realize — "Hey, nothing’s wrong anymore!"