My 3 week-old daughter developed a large bumb on the top-back part of her head. i was wondering what that may be? i haven't done anything to treat what she has because i have no clue what it may be.

Depending : Depending on the exact location, what it looks like or feels like, it could be many things such as: - a lymph node - an insect bite - bruise from a trauma - infected hair follicle - ringworm of the scalp - a cyst and those are just a few possibilities off the top of my head. If you are concerned or if it bothers her or is worsening, you should have her seen by her doctor. An examination should give clues to the proper diagnosis and treatment. Good luck!