GIRLS need your help?

When you show a guy alot of signs you like him, 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, then turns around and says " I have a boyfriend so PROBABLY not good going out" line, 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 ..