What Causes Numbness In Lower Lip And Chin?

I am experiencing numbness in my lower lip and chin. speech is not affected and the numbness is not on one side but rather includes all of my lower lip and chin. I have been to a doctor twice and have taken Ciprofloaxin to treat a UTI. I don't appear to have a sinus infection ...just wanted to see if i might get some other information on what might be causing this. I have had this for about three weeks now