Hi. I've been wearing a retainer for about 6 months. Its a plastic retainer I got after my braces were taken off. I have to wear it for about a year in total to stop my teeth from shifting. I wasn't wearing it for about a month and when I put it back in, it felt like my teeth had shifted but after about 10 minutes or less of wearing it, it fits fine in my teeth. Why is this? Why does it feel tight in my mouth for the first few minutes but then feels normal after that?