Do girls change when they have bf's?

I'm bestfriends with the girl iv fallen in love with and we have become really close. She tells me everything and would always be. There for me like I would for her and we used to msn each other everyday to every other day. I'm really shy around her but she doesn't seem to mind in fact she thinks I'm hot, sweet and a nice guy that shouldn't try to impress her. I told her to dont ever change for everybody because I love her the way she is and she promised me. We used to watch movies together, play cards, video games etc and she would sometimes smile and stare at me so I feel that she might have feeling for me like I do for her. Now she has a bf of 11 months and everything has changed, we hardly see each other nor talk and iv tried seeing her but she's never around because she's out with her bf or working. I only see her once a month rather than everyday like I use to but when I do try to see her I buy her a dozen roses even when she is not home and leave it on her bed. She told me that she has with him often which made me really upset because all her cares about is but she doesn't see that which basically forced me to tell her how I felt about her. After I told her she didn't freak out but all she said was I see and she couldn't do anything about it and she understood that I didn't want to ruin our relationship. Since then she barely answers my texts and we hardly talk but we are still very close yet I feel so far away from her i kiss her on the cheek sometimes and she would say "awww thank you" or would smile and look at me but she also talks less about her bf and she sometimes says or done stuff that is out of the ordinary. When I visit I only talk to her for a few min out of a thirty non period then end up leaving. I don't know what to do, she is my best friend who I never had this feelings for any other girl before and I don't want to give up.