I know how utterly infuriating this can be, and I'm sorry that you're going through it. It feels as though you can do everything right and still have your credit absolutely screwed through no fault of your own.
I was involved in a bike accident in May of last year that left me with a concussion. I visited an ER and got checked out. I paid my co-payment, provided my insurance information, and paid every charge I was responsible for on time and in full. I had interned in a doctor's office in high school and know how often medical billing departments screw up, so I was very proactive in making sure that everything was squared away. I even called the hospital and my insurance company a number of times to make sure that everything was on track. I made sure everyone got the right information and that the hospital bills were taken care of, and confirmed a number of times that I had no outstanding balance with them.
What I didn't realize at the time is that many hospitals use third-party staffing companies which provide their ER physicians. In many cases, the doctor that you see at the ER is not actually a hospital employee. In such cases, the medical staffing company will bill you separately. Calling the hospital and ensuring you don't have a balance with them doesn't necessarily mean that you don't have a balance with the company who provided the doctor who actually saw you.
I moved a few months after the ER visit, well after everything was taken care of. I didn't find out that there was a problem until one day I got a bunch of mail that had apparently been sitting at the post office. In that delivery, I got a notice that the account had been put into collection, as well as a past-due notice on a $1000+ dollar balance from the collection agency. I never got a bill, just the notices from the collection agency.
To make matters worse, I've since found out that the charge being reported is not even correctly billed, nor something I should be responsible for. They're engaging in a practice called balance billing - coming after me for the balance that my insurance company didn't pay. It is, in most cases, wildly illegal. The damage to my credit has been substantial, nearly a hundred points. I barely even know where to begin unraveling this mess.
I graduated from college with well over $100k in student loans. I have spent several years working my ass off to build my career, develop a good credit history, and get my debts under control. Even though I have a reasonably high-paying job at a company you've almost definitely heard of, I've deferred home ownership, made do without a car for several years in a city that pretty much requires one, and sacrificed some potentially incredible experiences so that I could pay my debts swiftly, efficiently, and without failure. I was looking forward to buying a car later this year and perhaps my first home the next. All of that may now be in jeopardy.
tl;dr: If you visit an ER (or any medical establishment), be absolutely sure that you know every minute detail of your billing process - what you're being billed for, who is charging you, and how it's being settled. Don't trust medical billing departments. There is a very good chance that they will fuck up your financial life if you give them even the slightest opportunity to make a mistake.