The inability to stop smiling because of someone you are romantically interested in/involved with. Both men and women suffer from Girlface regardless of whether they are interested in a man or woman.
A: Why are you smiling?
B: No reason...
A: You have girlface! Why do you have girlface? Who's giving you girlface
B: <evasive> Um...
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.