Calli, I don't think you're crazy. You're hypervigilant because your last close relationship chemically taught your brain to go into overdrive.
Most social media sites are very subtle about blocking these days. I'd just block that strange account. Your emotional wellbeing is far more important than some stranger being able to see your posts. On the off-chance that it's actually someone you know well (and is good for you), they'll likely shoot you a message, tell you who they are and ask why they can't see your posts anymore. So there's nothing to lose by blocking them.
If blocking seems too extreme, you might consider hiding your stories from them specifically (can be done in the settings).
And yes, this
will stop. It's hard now, but the darkest hour is right before the dawn. You'll get there.
