Hi ~ I'm sorry you're having so much pain. I know it's frustrating.
You are still healing from your fusion. And many nerves were injured during that surgery. Nerves heal at the rate of one inch per month, so it takes awhile for them to recover. It could be nerves regenerating that's causing that burning pain.
Also, it might be that you're using poor posture. That's what it sounds like. Do you find yourself 'kind of' slouching a little when you sit?? Even a little bit can cause that kind of pain. You have to remember to sit up STRAIGHT and that's hard for any of us to remember, but it's crucial. Try to sit straighter and make sure your posture is good from now on.
If, after a reasonable length of time, this doesn't clear up, see your doctor and ask him about it. Chances are he'll tell you the same, but hopefully he'll run some tests to be sure nothing is going on in there. Best of luck and God bless.

Hugs, Lee