It sounds to me like she is nervous around you. She seems to be trying, at least, to associate with you, but I'm guessing she is afraid of stepping on toes or saying/doing something that might set you off.
Is it possible that she has had a precious poly relationship where this sort of thing has happened? Where the primary female partner said she was ok with the dating and then flips out?
You could be straight foward and just ask, like LR said. That may be the simpliest although difficult thing to do. I wouldn't get the boyfriend involved though at this point, might make it seem like he has to take sides or some such.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.