When do you consider intrusive behavior to be "stalking"?

Warning: Yap session imminent

One of my girl acquaintances knows my League of Legends handle and the name I used to compete under for fgc tournaments. That's cool and all, but then I realized that this info isn't on any of my socials anymore, and it's over 5 years old.. especially my League name. That account is banned, so there's no way I played with anyone on that account for half a decade. Not a single one of my IRL friends plays League or even knows too much about gaming. I tried to ask her how she found my old tags but now she don't wanna respond... she has also been very persistent in trying to get into my Twitter/Instagram to see my liked tweets and such. I just starting think about this too, but she also knows where my office is and is in regular contact with my online friends as well.. I've only known her like 4 or 5 months. I'm not really used to people being this invasive, but I don't want to call it stalking if this is actually normal and I'm the out of touch one. What do you all think?
When do you consider intrusive behavior to be "stalking"?
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