GIRLS why she acting like this?

When you show a guy alot of signs you like him then do this what does it mean? this girl I know likes me has done things which in the past other girls that have liked me have done and asked me out, she always blushes when we make eye contact, staring when im not looking, open body language, staring right into my eyes after we talk and smiling, when we stand together shes always looking down and smiling and always smiles as we talk .. then when i asked her out she then turns around and says " I think you should know i have a boyfriends, been together a while so PROBABLY not a good idea going out for a drink" is she telling the truth even though she has shown good signs and no negative body language .. Is it a test? what should I do? Or is it an outright slow let down .. It's been over a week since I've seen her after asking her so don't know what she's feeling, but now it's like she's avoiding me .. I genuinely like her and want to get to know her .. Shes not a flirtatious girl from what I know ..