It could be fat necrosis in the breast tissue. Fat necrosis is a condition in which the normal fat cells of the breast go through a change and become round lumps. The lumps may or may not be painful and may be firm. The skin over the lump may be red or look bruised. Fat necrosis may occur after a bruise or other injury to the chest or breast and can occur from weeks to years after an injury. Fat necrosis usually goes away without treatment but can form permanent scar tissue that may show up as an abnormality on a mammogram.
Another site I visited said it usually clears up on its own within a few weeks. Since this has been a problem for over a month now you may want to have your doctor look at it and possibly do a biopsy just to be on the safe side.
Be well.