It is possible that the nail is starting to grow under the skin and will work its way out as it grows. Many times, if you damage the cubical bed, the nail will have a difficult time growing if it even does at all. Give it some time and see what happens, if it begins to cause pain or the lump grows under the skin more than a quarter inch, talk to your doctor about it.
The nail grows and s connected to the skin. If you lost your nail it grows back it may appear to be growing under the skin but if you look at your other nails are they not connected to the skin? The nails grow in areas that are used and need the protection of the nail. The nail protects your fingers and toes from as you found out getting slammed. Imagine what would happen if your finger was slammed and you didn't have a nail. Or, if your toe was stepped on without the protection of the nail? Yikes!