What you are describing sounds like costochondritis. It isn't necessarily related to PMR but it is possible.

"Examples of health conditions that can feature costochondritis include fibromyalgia, psoriatic arthritis, ankylosing spondylitis, reactive arthritis, rheumatoid arthritis, lupus, and inflammatory bowel disease (such as ulcerative colitis and Crohn's disease)."
https://www.medicinenet.com/costochondritis_and_tietze_syndrome/article.htm

I had PMR as an add-on to reactive arthritis. I often had flares of costochrondritis when it hurt to inhale every time I took a breath.

I'm now going back into history as this happened only once, and it must have been 40 to 45 years ago. It was at a time when I had an inadequate doctor and ended up changing to the best doctor I have ever had. He didn't know what it was but suspected autoimmune problem and gave me the only test available at the time, for Lupus. Although the results weren't conclusive, he suspected Sjogrens Syndrome and prescribed hydroxychloroquine which I took for many years. I believe I have had PMR for a very long time, and his intervention prevented a flare up happening much earlier, and kept me safe at a time when medicine didn't recognized what PMR was, also as a single parent, when I needed to earn a living.

Now I am retired and taking 1 1/4 mg of prednisone and about to come down another 1/4 mg. I know it sounds like a ridiculously small amount but after 2 years of trying to drop below 2 1/2mg it works for me.

To get back to the topic, I relieved the pain in my ribs by gently massaging between the rib bones with my fingers. Keep smiling sunshine, it does get better.