Mustang Filly Question?

I have a 5 month old mustang filly, she just got to the point where she will let me pet her and rub her neck etc, but when I'm done and remove myself out of range, she sticks her head through the round pen and tries to nibble on my hoodie or pants. I'm not sure if she is just being mouthy or is trying to "groom" me. Either way what should I do about it?