I personally wouldn't think it's a big deal, if I had just dated them for a few months years ago. However it sounds like there is more to it than that. You and this former friend were fighting, she was mad at you, there was weird passive-aggressive drama involved, etc. It sounds like it's more about the matter that you and her's friendship was already on the rocks, than about any one specific detail. I would imagine (could be wrong, of course) that if she was a very close friend and things between the two of you were great, then maybe her dating some dude you saw for a few months years ago wouldn't have been a big deal. But if you felt like she was only doing it to "get at you" than I'd imagine the motive itself - to "get at you" - would have been what was hurtful. Do you think there could any truth to that?

I had a very close friend for a year in high school, and she was a lot more attractive than me, so it was typical that a guy I crushed on or tried to hang out with, would end up going after her, instead, and she clearly enjoyed the attention. But because this girl was very special to me and I really enjoyed and cared for her, the guy issues didn't bother me because they paled in comparison to how much I cared for her. However I might have felt differently if she wasn't a true friend and just some girl in my life who created drama.