Basically, it is your scalp adjusting to its new pH. Some describe hair as feeling heavy or greasy during this interim. Your scalp will try and figure out how much natural oil your hair follicles really need to stay healthy. If you were using anything with a harsh detergent or foaming agent (think sulfates-- like Sodium Laurel Sulfate (SLS)) transition period is possible when switching to a natural shampoo bar. Commercial shampoos/conditioners almost always contain synthetic silicones that make hair shiny. This builds up residue which takes time to get worked out.