The lump over the forehead after a fall is due to accumulation of blood & it is called hematoma
The blood cells accumulated are eventually broken down and absorbed in the body.
But this is a slow process & takes times to disappear completely.
Till that time, symptomatic relief is obtained by taking analgesics, ice compression & adequate rest.
If it still persists or there are any other associated symptoms like vomiting
, loss of consciousness
, convulsions, etc...then it will be better to consult your Doctor.
Wishing you a good health.
Thanks & take care.