I, and some of my friends, have the similar experience as yours before. I think Martin007 may be right: “也许你的牙医想你常常回去探望他呢”. “你在补牙前钻(磨)牙表面的烂牙时候也许钻得太深而钻到牙根里面去了”. If it is so, you need to do a “root canal”. It’s very expensive, and the student’s insurance only cover 80% of this expense. So I went to another dentist. If you need some information, you can e-mail me to opal88z@hotmail.com.