Hi and welcome to the Chiari forum.
I agree you should have another MRI as scar tissue may have formed and could be causing the same CSF restrictions as prior to your surgery.
Plus, I am not sure if 21 yrs ago they did testing to check for ALL related conditions....do make sure you do not have any of them as they have the same symptoms as Chiari.
But 21 yrs post op....that is great !! How long did you go b4 you had symptoms return?
Try to get your Chari looked at again by a ns. Unfortunately surgery isn't a cure but it helps tremendously with symptoms. You may have some of your chiari back. If it's allergy related sometimes that can cause severe pressure also. Your best bet is to talk to your doctor. Could be something as simple as changing your allergy medication. Myself personally I have horrible sinus and allergy issues and have tried everything. Finally I tried Flonase and my symptoms majorly improved! Best of luck to you! I am so sorry you're dealing with that horrible pressure pain, it's just the worst!