A retainer should always fit properly and not be too tight.
When you don't wear your retainer for the amount of time the dentist requires you too then the retainer can become too tight or not fit at all because of your teeth moving back to their original positions.
But if you've been wearing your retainer all the time like you're supposed to then it suddenly has become too tight then the retainer may have gotten warped or has become damaged in someway.
A retainer needs to fit properly for it to work like it should as you could damage your teeth when you wear a retainer too tight so I would recommend seeing your dentist about the retainer as you may need to get a new retainer or a better fitting retainer.