In my novice opinion it does look like a small rip or torn flap of some kind but doesnt nessecarily need stitching and doesnt appear to be bleeding...strange. And you say shes not rubbing it at all? I will be interrested in hearing what the others think?
Me, I did have the vet out after Peanut foaled but that was because it was my 1st foal and I just needed some reassurance that all was well w/both mom and baby. If the vet was coming to you anyway I would most certainlly have them take a look see but maybe the others can suggest something to put on it or maybe if shes not sore back there and everything else is fine as long as it stays clean it will heal on its own.
P.S......had to clean the computer screen off as hubby blew his coffee all over it when he saw the picture..." What the heck is THAT"...I told him not to read over my shoulder.
On another note, your filly is just darling...love her markings..she'll be as pretty as mom Im sure. Again congratulations!! It was pouring down rain here in Ft.Myers when she was being born...Im so glad you were not having thunderstorms where you are...I was thinking of you! Glad all went well