What is the other post, the first one? Who posted it?
Can you post copies of the PMs you got? Also, here's another thing you can check: When you open up your profile, and hit "transparency", click "show me my transparency page". That will show you all your posts that have been hidden.
Two things I can think of that could be happening is, somehow you're getting dinged for someone else's posts or, if the hides don't show up in your transparency page, you're just getting someone else's PMs.