Is she into me, Im receiving mixed signs?

There's a girl I like at work. She seems very shy and I've tried to establish some communication with her. I've seen her talk to other guys like nothing even joking and laughing. With me she is different, get nervous when I approach her, and our chats are small and mostly questions. The other day she went to ask for some material to my department and she was talking to my mate, I approach them and he told her I was behind her and she said "oh no, not him", the guy told her I'm a ghost cause she didn't notice I was behind her and she said "yes he is" and immediately said to me "no, I'm not serious". I felt that as a rejection, as if she was looking the other guy to handle her the material she needed. So I don't know what to think, I really suck when it comes to understand women behaviour.

Is she into me, Im receiving mixed signs?
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