Think of owning a car and borrowing one; if you own it, then it's yours to enjoy ... if you borrow it then, at some point, you have to take it back It isn't yours, in short.
Liking a girl/lady is a lot different from being in love. She has the right to flirt if there is no real bond between you guys; from what I see there isn't. She says she likes you, but consider she could like a few others besides you.
From what you say you are amongst others that she would say she likes ... she doesn't actually belong to you and only you. My take would be that you chill-out and not let it bother you. If she wants to make what you have a relationship, rather than a friendship, then things would be different.
As it stands? Again, and from what you say, she is simply a friend ... no matter how good a friend she may say she is. She isn't commited to you in any way.
Good luck
~Frankie~