My Page rank goes down

What do you think why the Google page ranks will go down? Yesterday I was surprise that may page rank goes down from page rank two to zero, it was frustrating. And first reason that we should pay attention is the difference between the paid post and not paid post. I noticed that since I accepted lots of reviews articles or paid post my Google page rank was affected, the more you post a paid post the more it goes down since Google did not count any sponsored post on your blog, you will notice that the paid post usually won’t crawl from the search engine. I think whether you have more readers or visitors it doesn’t matter since the content article is the king. Well the best thing to do since the content of blog is the king, make more relevant unpaid post than sponsored post or paid post. I have learned that your page rank will change anytime.