Retention phase after Braces treatment is Very Important. Relapse chances are maximum when you don't wear the retainers properly and sincerely.
it will take you 2-3 days to get adjusted to your retainer. Not to worry, as you got adjusted for braces, you will get adjusted to the retainers too, soon.
If it is hurting you visit your doctor and get it corrected. If not he will explain you the cost. You dont need to change the retainer until it is irreversibly damaged.
Usually single retainers are adjusted throughout the treatment.
Health Tips
Keep Patience. And follow the instructions given to avoid relapse of the treatment.