I would definitely get an MRI as soon as possible. Sometimes if you tell them you're in pain and need this, they will give you a quick appointment. Then it takes 2-3 days for them to gather the results. Try to make a doctors appointment for as soon as possible after that. I'd also try to get on some pain medication, it really does make a difference and should definitely allow you to move easier. Since you seem to be in a good deal of pain, I'd try calling your doctors office tomorrow to see if they can call you in some medication so you have it until you see him. If you think you can deal with the pain, then wait until you see the doctor, but there is really no reason to wait in pain. Being in pain and waiting for an appointment to come, especially one that isn't anytime soon, is a hard thing to do so try to avoid it.
As for it being a pinched nerve, I had a pinched nerve and it mainly caused pain in my left leg. If you have any leg pain, that could be a sign of a pinched nerve.
As I said, the best thing for you to do is get on some pain medication and get an mri. The longer you wait, the worse it will get, and the harder it will be to get an appointment. Trust me, I've had three back surgeries and still no success, waiting too long can turn it into an even bigger problem.