What kind of nurse licks an injury? I mean, it was a good way to get to lick her finger and all, but she still loses pretty much all her credibility as a nurse.

That's a very good point. I have to assume she does it entirely unthinkingly because of who this woman is and her suppressed feelings for her.

http://tvtropes.org/pmwiki/pmwiki.php/Main/FingerSuckHealing

Feels bad when you keep denying on what you feel and your trueself