From her age I'm guessing that this is pretty early in her relationship life. Depending on how she has been brought up it can take years to understand what you really want. In your teens (and early 20's) part of your growing is learning what you want out of a relationship. I think that her wanting to flirt with others is likely a safe way to try out this new idea of poly that has just been sprung on her. Maybe it will turn out that she is poly and maybe it will turn out that she isn't. The process of discovering yourself can take years. If you like her and enjoy her company just go with it and enjoy the relationship for what it is.
Everything will be ok in the end. If it's not ok it's not the end.