Any lump caused after trauma is not that serious.Mostly,it could be Haematoma
,collection of blood or traumatic fat necrosis
,a condition occuring in breast fat cells after trauma.
If the lump is not subsiding,please get it examined and go for evacuation or drainage
of the content.
Till then take an Antibiotic like Amoxycillin
and anti inflammatory drug.
Wish you good health and speedy recovery