Thank you for your query on Healthcare Magic.
If the lump is there since birth, you can think it is due to meningocele
If it is of recent onset, it could be due to a trauma which caused contusion
in her scalp layers.
As CT scan
is normal, nothing to worry.
Observe for any progression in the size and if you find any increase, consult the pediatrician
for follow up.
Hope I have answered your query.