Maybe your girlfriend meant to joke about your whole face in general, then only realized later that it could have sounded bad when it seemed like she was referring to your skin. It could be that since skin might not be an issue for her personally, it might not be on her mind as much, and she would not realize how bad it might make you feel. We are all probably hyper-sensitive about our skin, but it is only because it is something that we are self-concious about. I know we've all been in situations in which we've put our foot in our mouths for saying something that we realize only later we wish we could have taken back. I'd guess that she really didn't mean anything malicious by it.
About the dentist...have you ever seen how bright that light is that they shine over your head?? That thing is brighter than a thousand suns, man!
It probably made your scars appear much, much more apparent than they usually would, in normal light. Hope this helps you, a little bit!