Since this gal is your best friend, I'm surprised that you haven't already had some kind of conversation about your goals and intentions for next year. By all means find out what it is that is bothering her. Just listen- maybe something from the past about being left behind is what has her reacting this way.

I would also suggest that perhaps you set aside times you can both ride together and other times when you'll be riding either by yourself or with the guys. That way all of your actions are above board.