It could mean a few things. Maybe they're still curious about your life. Or perhaps they miss you a little bit and this is their way of staying somewhat connected without actually reaching out.
It depends. If you've moved on completely and don't care about their presence in this small way, then no. But if it makes you uncomfortable or stirs up old feelings, it could be a bit concerning. However, it doesn't mean they have bad intentions. They could just be nosy. You should focus on your own feelings and decide whether you want to allow this or not.
It could mean that they still have some level of interest in your life. Maybe they're just curious about what you're up to now. It doesn't necessarily mean they want to get back together though.
Not necessarily. It could be just a casual thing. She might watch stories from many people. But if she always watches yours quickly or interacts with you about them, it could be more significant.