Just got a bill from the hospital even though they said it was covered

I'm trying not to panic but I opened my mail today and there it was, a bill from the hospital for the surgery I had back in January, almost four thousand dollars that I definitely don't have. When I had the surgery, the insurance adjuster told me everything was approved and I didn't need to worry about anything. Now suddenly I'm getting this bill and when I called the adjuster she said she'd look into it but I haven't heard back and it's been three days. I'm terrified that they're going to send this to collections or that I'm going to be stuck paying for a surgery that was supposed to be covered. I don't have that kind of money sitting around, I've been out of work since I got hurt last year and the workers' comp checks barely cover my rent and groceries. I know I need to get on top of this before it gets worse but I don't even know where to start. I've been trying to find a NY workers comp claim attorney who can help me figure out why this bill came through and make sure it gets paid by the insurance company like it's supposed to. Has anyone else had this happen where you got a bill for something that was already approved? How did you get it sorted out? I'm already stressed about my recovery and now I have this hanging over my head and I can't sleep. Any advice would really help right now because I feel like I'm drowning.