Question About Wordpress Categories

I have a website called DietReviews101.

I was ranking on Google on page 1 and 2 for my main keywords.

On Saturday I added several new categories and added all my posts to additional categories so I could better categorize them for users.

Then today I noticed Google reindexed my site and it has dropped maybe 6 positions or so.

Now the permalink structure I use is /%postname%/ so even though my posts are in several categories, it has not created duplicate content as can be the case if you use the /%category%/%postname%/ permalink structure.

So my question is, how could adding some categories and making posts into more than one category cause my rankings to drop?

try the following permalink structure /%category%/%postname%. Just be sure that the category names are proper

You don't want multiple categories with that permalink structure as it creates duplicate content.

I wasn't asking about permalink structure at all.