Well, based on your story, maybe you just saw the movie "Desperate Housewives" or the somewhat the movie got stuck inside your head before you go to sleep, or you were thinking about it before you go to sleep.

anyway on the figurative side, the dentist you dreamt would signify that you are having doubts towards a person, and you fear that the person might be putting you in danger, or giving you wrong directions. and the numbness would signify that you let your fears control you.

on the physical aspect, sometimes when we feel uncomfortable, especially when you sleep and your arm is under your body, eventually blood circulation decreases that causes numbness. this numbness will trigger the brain, thus making you dream, and telling you that there is no enough blood circulation in your arm. these are all involuntary. there are a lot of things that our brain can do and cannot fully understand how.