Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concerns..
As per your complain it seems that due to traumatic injury
to the head caused due to hitting with fireplace mantel has caused injury to the soft tissues in the area as well as there seems to be trauma to the superficial blood vessels leading to spillage and accumulation of blood into the soft tissues leading to formation of a lump..
Pain is due to inflammation..
You need not to worry as it will gradually resolve..
As of now you can start doing cool compresses over it..
Apply numbing gel over it to reduce pain..
Take anti-inflammatory painkiller like Ibuprofen
You can apply Heparin
gel as it helps in faster disruption of Hematoma
In case if it still does not resolve consult a General Physician and get evaluated..
Hope this information helps..
Thanks and regards.
Dr.Honey Nandwani Arora.