that does look pretty bad.not life-threatening, but definitely a rough one. i almost feel like i've seen this type of injury before but i can't seem to recall. i'll try some google-fu.
are you positive there's nothing she could hurt herself on? i've thought the same thing only to find Yellow with a huge red mark on his face and no clue what happened, but he's had more injuries than i care to admit; i think we're at 4 or 5 injuries.
TBH i'm not sure what course of action you should take. i'd keep an eye on it and if it gets even slightly worse i'd take her to the vet.