Hard lump in mouth after wisdom teeth extraction?

I had two of my wisdom teeth removed a month ago, & since then I've noticed a small hard lump near where they were removed in the lower part of my cheek. It's not a large problem, but it's somewhat restricting in how wide I open my mouth(It hurts a little when I stretch it) & it's just bothersome feeling it there. Does anyone know what it could be? I read somewhere that it may be a bony callous but they said that that should disappear after 3 weeks.