Did you do any investigation for that? Like ultrasounds? (If not would be the case to have one, ultrasound are quite cheap and are perfect to diagnose any problem in such superficial structure)
Did you try to ice it and gently stretch your extensor compartment?
which kind of exercise did you use to do?
And how come did you have this operation? Did you have pins and needle on the back of your forearm or back of your hand?
Is that the only point where you feel pain or have you got any other associated problem?
A part from typing can you think to anything else that make the pain worse?

Sounds like an overuse type injury on your extensors muscles of the forearm.

All the best