Well depends how well you know her... have you spoken before? Personally if a guy checks you out and initiates talking to you then its clear his intentions, but if a guy just keeps staring continually (also do you smile or not?) then it may just come across kinds creepy or she may think you have a problem with her if its not a friendly stare and then thats why she might start acting hostile...haha sorry that sounds kinda weird but you get what I mean! Maybe give a bit more info on the situation and we can try help!