There could be many reasons your parakeet started pulling out it's feathers and then scratching the feathers off it's head.  Not all of them are related to physical maladies.  It is just as likely the reason is psychological.  What has changed in your birds world?  Has anything changed in the amount of time you spend with your bird or in the quality of time you spend together?  Have you moved it's cage recently, or changed the kind of food it eats or the time of feeding?  Have you added or removed another person or another pet in your household recently?