Is it normal for a best friend to lie about her bf?

Well my story goes like this:
I knew a girl since my junior college days, after passing out college. We both met after 8 long years.
I got to know her more as a person. Started liking her. Days passed and our meetings increased, we discussed our past relationship often. One day, I fell in love with her realising the person she is. I didn't have much courage to tell her the truth but after discussing my problems with my cousin I gathered courage and expressed my love for her one night. She rejected me saying I'd get better girls in life and she is not ready for a relationship.
Some days later I came to know from a trustworthy source that she is already in a relationship with a common friend alongwith photographic evidence. I asked her to confirm. She denied by saying that they are just friends and I should think that way.
During the festive season, she called me over dinner. I picked her up from her hospital and took her home. We had dinner and then she called up the common friend (with whom she's in a relationship); he too came for dinner.
I could sense some uncomfort from his side. I asked her why was he uncomfortable the other day to which she replied he's always in his own world.
Why does she behave like this? I am not good enough for anyone?
I'm currently in a state of depression.. pl. advice