night guard replacing retainer?

I had braces almost ten years ago and have been wearing a retainer regularly since then. I was recently given a custom fit hard plastic night guard by my dentist for my grinding at night. Is it true that this will keep my teeth in place the same as a retainer? I've noticed since wearing the night guard the past couple of weeks that my front two teeth feel like they move when drinking water and I have to push them back forward with my tongue. Is that related at all to not wearing my retainer or potentially effects of the new night guard?