Hi Happygirl!
I'm not a stepmom but your post raises a few thoughts in my mind. Being that she's 16, maybe she feels she has to choose between you or her to treat as a "Mom". There's also the possibility that she thinks you're hurt by the amount of time she's not spending with you and spending more with her Mom. So, she's backed off a bit to spare you any hurtful feelings without realizing that it hurts you more.
IF she's in limbo between 2 Moms, and I do mean IF just in case I have it wrong. I have those moments where I am dead wrong, and I admit it. :-) Why not try a different approach? How about reverse psychology? Or a more positive spin on it, as I always say. Instead of feeling hurt, feel glad that she's reclaiming some lost time with a woman who needs her as much as your stepdaughter needs her. Tell her that you're glad things are going well for them and encourage her to do more if she can. This way, your stepdaughter realizes that there's no competition over her. Just 2 women who love her very much and want the best for her and be a team of sorts. You don't have to like her mother, but since your stepdaughter needs both of you for different reasons, support and encouragment is always helpful.
Just my 2 cents! :-)
Mary