Forcing a retainer back on

I have not worn my retainers in over a year, however, I can slowly slide them on, little by little. When I do take it off (Usually can't stand it for longer than a few hours since it has been so long), my teeth and gums hurt or are sensitive for an hour or so to the touch. It was like I was wearing braces again after a tightening. I was wondering if there can be any damage done by wearing a retainer that is making my teeth shift significantly back into place? I know there is discomfort and pain afterward and was wondering if anything internal can happen that should not be if I keep wearing these retainers?