Thank you for your query on Healthcare Magic,
The knot on her head closer to the base of the skull may be a small enlarges suboccipita lymphnode because of local present or a previous skin infection
of the scalp
It may be palpable as a firm, round swelling.
Nothing to worry as there is no any discomfrt to the child.
But in next well baby visit you can discuss with your Pediatrician
about the knot like swelling for examination and its nature.
Hope I have answered your query.