This could be a bunch of things-- It likely is the extensor retinaculum problem. In the extensor retinaculum cases, the weight bearing is oddly affected but only at the higher ranges of weight bearing. For instance-- if your age is 21, male, 6'1, normal weight bearing should be 100 lbs. You will not see the extensor retinaculum issue until you are at the 80-100 lb range. You can also test this by making a fist with the long finger tightly flexed. Then flex the wrist into wrist flexion and while the hand is flexed, lift the long finger against resistance.
Give it a try and let me know what you find.

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Get some tape on the wrist or the wristwidget and see if your pain goes away. You cant do the weight bearing test this soon after surgery but it should be a part of your post op care. The TFCC is related to the ECU and often if you have a primary TFCC injury, the ECU takes on a load and becomes irritated. Make sure your post op brace does not compress the ulna as this is a common problem that impairs the healing of the tfcc and causes a ton of pain.