I think you've got demoted since the forum is full of trash posts so maybe the moderators are deleting some thread that is useless and trash post. I don't say that i am a perfect member here since I am also one of those who decreasing its posts. hahaha! My posts are also decreasing and I've read on the Meta section that the moderators putting some thread on the trust and I think that is why you've got demoted because you've lost some of your post.