Does wearing my retainer straighten my teeth?

I got my braces off about two months ago and would wear my retainer day and night for the first month or so until recently i have only worn it in the night though my orthodontist said i should wear it all the time for a year. The retainer still fits. Every night when i put it in it feels a bit tighter but loose in the morning. Does this mean my teeth shift back to being straight during the time i have my retainer in?