Scalp swelling in most of the cases is due to head trauma
may remain unnoticed in many cases and then swelling will be the only manifestation.
Scalp swelling following trauma may be soft in consistency as in your case.
Other cause of soft scalp swelling may be a cyst, abscess
Out of these only cyst will be not painful.
Since it is not present since birth so congenital causes are excluded.
I would suggest you to consult your doctor for clinical checkup to find the diagnosis.