I work in a factory and I'm on my feet all day bending over a conveyer belt. Because of my job my back is now seriously messed up and I've had to miss a few days of work for doctors appointments and therapy in order to get a handle on the situation. Then, what do you know, my boss called me into the office and told that I needed to try to make it to work regularly. Keep in mind I've always had a doctor's note when missing time and we have a company policy that states it's an excused absence if there's a doctor's note involved. When I brought that point up he replied, "Excused or not you need to work on coming in regularly." Now what?
Screwed Up and Pissed Off
If you haven't already done so,start documenting everything relating to your back injury; doctor's visits, doctor's notes (make copies before turning them in), test results, therapy schedule, etc. Also, find your employee handbook and make a copy of the section that states "absences are excused with a doctor's note", and hang on to everything. North Carolina is a right to work state which means an employer can fire an employee at any time without any particular reason, but you have a lot of ammunition to take to the state if they fire you after you suffered an injury on the job. In the meantime do a little on-line search about Workman's Comps claims. I'm not quite sure if repetitive motion injuries would be something you'd file a claim for but it's worth looking into.
Sorry about your back!