Washing hair after giving birth.

My daughter is 15 days old.. I just asked my mom to take me to the store to get some shampoo & she responded, "no you can't wash your hair yet". When I asked her why she told me to call my grandmother. She said the same thing and said idk why but it worked for me so it'll work for you.. -_- Can anyone tell me if there's a legitimate medical reason I shouldn't wash my hair for 6 weeks.