This was a few years ago for me but I remember that the pain I had eventually went away over time, specifically about 3 to 4 months after the pneumonia/fever was gone. During this time I was still active, walking and stretching mainly (no intense cardio).
Your description is a bit different than mine. I felt it when taking deep breaths, this is why I thought is was a muscle/tissue problem. I am not a doctor so I can't offer any opinion based on your message.
I know this was a long time ago, but I found your post when searching for "unexplained back pain after pneumonia". What was your resolution? Your description sounds exactly like mine, except that my pain is constant. No change with movement, deep breaths, or activity. I am desperate for an answer. I'm wondering if chest x-ray might have missed pleurisy or pleural effusion. The tech only took 2 x-rays, one with me facing machine, and one at my side. Wondering if she had taken one with me laying down on painful side (right), something would have showed. I am so frustrated. It has been 6 weeks since pneumonia, and I think I should be feeling much better! If any doctor reads this: I had pneumonia 6 weeks ago, had extreme back pain at time of diagnosis in my mid-right back closer to spine. 1 week course of levaquin. Back pain returned in same spot at 4 weeks post diagnosis. Lungs feel "funny" but I'm not coughing, and pain is not worsened while breathing. Severity of pain waxes and wanes. Chest x-ray at 5 weeks was clear, but as I mentioned, only 2 pictures were taken. Is it possible that I have liquid in the pleural space of my right lung that has become infected? If so, could that happen without a fever? Thank you for any input.
I know that you were asking for an expert's opinion, but I may have an answer for you. I get those sometimes and they seem to come from nowhere. I can't think of anything I did to cause these attacks.
I do go to a chiropractor on a regular basis to prevent degienerative disc problems and chronic fibromyalgia pain in my shoulders. He has been a miracle worker. The pain is not gone but my range of motion is almost 100% better.
One day I had one of those pains on the left side of my mid-back that ran to just under my arm. He told me that I had a rib out of place, adjusted it and I was good to go. It still happens every so often and he adjusts it and I'm fine.
Just a suggestion.