Is this girl trying to start drama with me and what's the point?
Cryptic Midnight Shadow
Block her on social media and ignore her. Maybe change your schedule.
Just curious, how have your coworkers responded to her strange behavior in the store? I'll second the idea of completely ignoring it. They probably already figured out she's a little "off." ...In the meantime, she's giving your store some extra business! :)
From you have said I get the feeling that this girl feels threatened by you and has somehow conjured up the thought in her head that your ex may still have feelings for you and/or that you could still him from her. I think the best approach is to not react to this girl's antics. As soon as you start showing her that what she is doing is bothering you then she wins - that is what she is looking for. However, if you remain calm and give neutral responses then this girl will get tired of annoying you and/or will more than likely self-destruct. I really hope that this helps :)
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Thoughts - your boyfriend obviously has talked about you with some fondness and she, being the immature little one, feels threatened that he has any sort of past and feels the need to defend what she thinks of her territory. Ignore her and maybe she will go away?
If the others are your mates word them up and get them to say hes rooted around on you and rooted the dirtiest whore in town after he broke up with you along with telling her she should be getting tested for various STD's.