You should go back to your dentist and let him/her know about it. Numbness may come from the extraction site or it may be due to the anesthetic injection. When we give shots, sometimes it can hit the nerve and cause some numbness until the nerve heals. The nerve also might have been traumatized by the extraction. It may take up to 6 months for a full recovery. Your dentist should be aware and should monitor the nerve to see how it progresses.