She does that often. She does not want Alex to see her naked. I cannot imagine what their "arrangement" is supposed to be.
She also seems to be very sneaky to me. Most of the time that she is there, she makes sure that she has her back to the cameras and hides what she is doing with her hands. I hate to say it, but I wouldn't trust her for a minute and I'm normally a very trusting guy.
Also, I don't like to stereotype either, but most people that I have known who wear black clothes constantly have low self-esteem, are angry a lot and feel the world owes them a living. I don't know if she is like that, but I haven't seen anything to dispel that thought either.