Dear user, When you get a hard enough blow, it causes blood to collect under the skin if there is no wound Hematoma, as we call it. That usually takes 4-6 weeks to go away, even after the redness is gone. it will usually change its colour from red to purplish to green and yellow before it completely fades away. If this continues to decrease in size, you don t really need to do anything. Just take pain killers and cold compressors as needed. If it becomes redder, or the swelling becomes more painful or increases in size, there are chances it has got infection. In that case you ll have to go your doctor to get it checked. In rare cases it needs operation to clean. There are very minimal chances that these kind of lumps persist after a few weeks, if it does persist though, you might need an x-ray to rule out small fracture, or any other kind of skin lump which you only happened to notice after the injury. Do write back if you need more help. Thanks