If your retainer still fits then that is a good sign. The pain will be your teeth moving back into the original position. It is strange because if your metal ones fit then your clear ones should really fit too. The only issue to worry about is if your retainer is to tight, it could push the teeth to quickly and possibly damage blood flow.
However, I think you will be ok. It is just like wearing a brace again, your teeth will take time to adjust. Remember, retention is for life. You should continue to wear your retainers at night even after 9 months if you want to keep your teeth perfect.
My answers and views are my own, not Quick Straight Teeth's