@isabelle7 One of the best things you can do is keep a journal with dated notes about your symptoms and if things are improving with PT or getting worse. That will tell you how fast symptoms are changing if they are changing. Keep track of what you are doing and body positions that can bring on pain symptoms. An MR is certainly expensive, and it could show you more detail of what you already know. If you had pain symptoms that worsened when you move or change neck positions, that might be the time to do an MRI. Your insurance company may not want to cover an MRI unless you have enough symptoms to warrant it, and your doctor probably needs to get a pre-authorizarion. Surgeons tend to think that an MRI is current for about a year. Sometimes the spine doesn't change that fast, but it also depends on your habits and if you play sports that pound on your spine, etc.

Adjacent segment disease does happen. It can happen after surgery for a fusion or artificial disk, and there can be multiple levels with issues even if there has been no surgery. If multiple levels have been fused, the risk of adjacent segment disease is greater because of greater stress. It doesn't happen to everyone. You surgeon can give you statistics on this. Generally speaking, the rate of adjacent disease is a bit more with a fusion over an artificial disk. Those have other risks too.

Having spinal hardware causes more rigidity on the spine. In comparison, my single level fusion was done without hardware and only a bone disk spacer. Bone has a bit of natural ability to flex. That may not be a lot in the spine. The other factor that I think is involved is scar tissue, and all surgery creates scar tissue that gets tight and it connects right to the operated spinal site. That can restrict movement, cause pain, and put pressure on the spine, and pressure can affect the spine. If you have a disc that is degenerating, pressure won't help that situation. Uneven pressure on vertebrae can cause remodeling of bone and can generate bone spurs. If you have a bulging disc that throws the force off in any direction, it can cause uneven pressure on a vertebrae. To ease this, I periodically stretch my scar tissue, and my PT does myofascial release work and has taught me how to self treat. I believe that this will help prevent further disc issues. I am 8 years past my C5/C6 fusion with no other issues. I was doing this MFR before spine surgery and it helped by making my tissues more supple and easier to retract, so it probably led to an easier recovery. This is my plan to avoid further spine surgery. My surgeon recommended maintaining core strength to support the spine as the best way to avoid further spine surgery. With stretching out scar tissue, I maintain better movement.

With all of this, I pay attention, and when I get tight, I make sure to stretch. With pillows, I have to make sure to keep my neck supported so it is not bent either up or down. If I get neck pain, I change position, and I stretch out the kinks.

Here is our discussion about Myofascial Release that you may be of interest.
Myofascial Release Therapy (MFR) for treating compression and pain
https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/
Has your physical therapist made recommendations on exercises to maintain core strength?

@isabelle7
I think you may want to get an orthopedic doctor to have you get a cervical MRI to see if your spinal cord is being affected. I would not rely on a PT to advise on medical tests and procedures. They are supposed to help you strengthen muscles and improve function, not diagnose. I had cervical spondylotic myelopathy c5c6 and had ACDF surgery. Myelopathy is spinal cord compression that can cause permanent damage/injury if not decompressed. No amount of physical therapy would have improved my spinal cord compression. Surgery was necessary to stop the further progression of arm/hand/leg weakness and loss of bladder control. I was diagnosed with this at 52. I am now 55 and have permanent damage due to my condition not being properly diagnosed over 3 years. I took myself to an orthopedic spine doctor who immediately diagnosed me based on clinical symptoms and MRI.