Workers comp approved my back surgery, then denied the claim saying injury wasn't work-related - what?

I don't fully understand how this is legal. I hurt my back at work in November moving equipment. Filed a workers comp claim. My employer's insurer - Sedgwick - had the claim approved, sent me to their approved occupational medicine doctor, that doctor approved surgery, and the surgery happened in January. Everything seemed on track.

Now I just got a notice that the claim is being denied and reclassified as "not work-related." Two and a half months after the surgery. The denial letter says a file review determined my injury was likely a "pre-existing degenerative condition" that was not caused or materially aggravated by my work incident.

I have an MRI from before the injury showing no significant findings. I have medical records documenting the work incident on the exact day it happened. My own doctor and their own approved surgeon agreed the injury was work-related. And they let me have the surgery before pulling this move. Now I'm apparently supposed to pay for the surgery myself because they changed their minds after the fact?

I'm in Georgia. Do I need a workers comp attorney immediately? This feels like it can't be right.