Replacement retainer is causing pain?

I got a retainer 2 years ago after my braces were taken off and have worn them every single night since. The top retainer began to crack from wear and I went to get a replacement. However, my upper teeth now hurt as if I had braces my tightened but I'm only wearing my new retainer. It seems as though my teeth are moving slightly, is this normal? My gums are fine, my teeth just ache.