There's not enough information that I'd be comfortable drawing conclusions about this. One person's past flame can be another person's one who got away. It's entirely possible she's keeping tabs on you online in a method you're not aware of, but if you don't know that she's intentionally moved to be close to you and she hasn't done anything concerning like made threats or faked a pregnancy or created circumstances that compel you to interact with her against your normal inclinations, I wouldn't guess stalking. Some people do coincidentally reconnect.
That said, the important question is whether you want to engage with her or not going forward. If you don't, I wouldn't lead her on by giving her any more attention. Make a clean break and just tell her you're not interested. If she reacts with melodrama or stalking behavior, then you'll definitely know you made the right decision.
If you are interested in possibly pursuing something with her or at least giving her a chance, be honest that you're a little freaked out about how she's previously behaved. You shouldn't proceed with her thinking that the behavior was not concerning. She should respect your comfort levels if she wants a relationship. If she's dismissive of your concerns and comfort, it's a big red flag that you shouldn't engage further.
I completely forgot about that game until you mentioned it. Fun times.