You have probably seen it posted here many times, when people break up, to stop following them on social media.
Who's to say that others weren't making comments to her like "oh PlantsVorganic514 was on holiday this week and went to such and such, looks really happy".
Your ex may have said enough, don't want to hear anymore, please stop reporting and maybe block her feed.
You just don't know, you are just guessing that she is bad-mouthing you. Not that the reason really matters since it's still hurtful to you, but is this a person you really need in your life? Have you asked yourself that?