Maybe she has her own issues that she's dealing with. Also, maybe she is afraid of getting hurt. Either way, if you think she did it just out of malice, then she's not worth another second of your fret.
Some people may have feelings for a particular person, but fear how they would be perceived by others if that was known to be true. If, for whatever reason, this girl's friends might see you as an undesirable choice, then she might be afraid of what they'd say.