After tooth extraction there is some swelling and inflammation at the site where the teeth are pulled out. As the cavity left by the tooth fills in, there may be a residual bump as the tissue matures and heals. It is also possible that the removal of a tooth typically leaves a sharp, prominent edge of bone at the rim of the socket. The gum can easily heal over these edges, the overlying soft tissues are vulnerable to injury if pressure is applied. Since the gum was cut at that particular site it is possible that it will pain for some time due to injury and inflammation.
I suggest that you return to your dentist to have your lump properly assessed, identified, and managed as needed. A clinical examination is very important.
Hope this helps. Keep me posted. Kind regards!