Lots of things
Depending on whether it's painful and your history with that ear, it could be a keloid (lump of scar tissue), could be a small cauliflower ear
, it could be an infected cyst or a big pimple.
If it is bothering (pain, itching, redness, etc) then you need to get it checked out. A lump should always be evaluated and investigated by a physician. Don't play around with it.