I?m assuming that you are all teens or younger. And I am not trying to sound wise, but maybe I?ll just sound old :-).
Anyway Fantasyfiend, that is common and normal to happen, people can take the wrong impression from anything. Also, you could be giving vibes of the shy guy wanting to get together, or she may have some interest in you, but not wanting to endanger her current relationship.
All in all, it can be 1000 different tiny issues, from miscommunication, to not wanting to invest in a friendship with you other then simply a passing conversation, and now she knows, you are more interested then that.
From my past there have been cool people that I liked that didn?t want me around all the time, but they enjoyed a conversation with me from time to time, as well as people who thought I was cool, yet all I wanted from them was a passing relationship. Everyone has different wants from different people, not everyone is going to be totally into you (in a friendship way).
Based on past experience and depending on how interested you are in perusing a relationship with this girl (whatever the nature of the relationship may be) confrontation is probably the wrong angle, it would be making an issue out of nothing.
Befriending the boyfriend is better, and your avoiding her for a while would be almost as good, if she confronts you on it, simply tell the truth, you thought she was put off by your asking her to the movies and you did not want her thinking you where some kind of creep.
But overall it seems like a small issue.
