The soft lump at the scalp
is likely to be haematoma
which is collection of blood.It will dissolve by its own after a few weeks and it may take 2-3 weeks for the swelling to disappear completely.
You should also have the x-ray of scalp done to rule out any scalp fracture
and consult the doctor for the same.The use of vitamin c
is also recommended as it helps to dissolve haematoma early.