There could be a variety of reasons. Maybe they miss you and want to know what you're up to. Or it could be a subconscious habit. It's hard to say for sure without knowing more about your specific situation.
It could be that your ex is just randomly browsing and happened to come across your stories. Or, they might still have some lingering feelings and want to keep tabs on what you're up to.
It depends. If you make your stories public, then anyone can watch them. But if you set them to private or restrict access, only those you allow will be able to view them.
You can check your saved files or folders where you previously stored them. Maybe on your computer's hard drive or in a specific cloud storage service you use.