Is this a possible relapse to PMR or something entirely different??

Relapse without elevated inflammatory markers is possible. Prompt response to
prednisone raises that possibility.
Testing in addition to usual
CRP and ESR may point to other conditions. Can you get a referral to consult a rheumatologist?

My PMR has been in remission for six years and I'm always thinking about it coming back when I have extra joint pain. Recently my shoulders have had a lot of pain and found out it's related to my degenerative arthritis and rotator cuff pain. I had some PT and learned a few new exercises that help strengthen the muscles and tendons in the rotator cuff area which have helped if I don't over do it. I've also found the Voltaren or other generic pain gels with lidocaine help when the shoulders are hurting. I've also used the pain patches on the shoulders which help.

You might want to discuss the pain with your doctor or rheumatologist as @seniormed mentioned since there are a lot of conditions that can mimic PMR pain.

"There are a lot of conditions that can mimic PMR pain."

There are a lot of conditions that respond rapidly to prednisone too!

PMR pain isn't the only condition that responds rapidly to prednisone. I can think of at least 5 other conditions in my own experience that had nothing to do with PMR but the pain was relieved quickly after I increased my prednisone dose.

When "systemic inflammation" is the problem ... pretty much any kind of pain will respond quickly to prednisone. The systemic inflammation and widespread pain wasn't all PMR but it made it easier to say it was a PMR flare to justify why I increased my prednisone dose.. This is especially true when PMR is a person's only diagnosis and nothing else is explored and/or considered.

I would play it safe and not blame PMR for everything. Most of the time I would tell my rheumatologist that things would hurt for no reason at all. Fortunately, my rheumatologist still believed me even when she didn't know why things hurt or couldn't come up with any rational reason for the pain..

"My shoulders are killing me. But and this is a big but the right-side hurts more than the left. The pain is on both sides, but the Right is definitely much worse. My hips only bother me slightly as well as my knees. But the shoulder and neck pain are awful."

No criteria that I'm aware of says PMR pain has to be bilateral and "equal" ... just bilateral.

I don't think bilateral pain is an absolute criteria ... just that PMR "usually presents as bilateral pain" in the shoulders, hips and neck areas.

Elevated CRP and ESR levels are good supporting evidence but not necessary and aren't specific to anything. My inflammation markers were almost always elevated but my rheumatologist never checked them very often. She was very surprised when my ESR and CRP levels were elevated on 30 mg of prednisone. I think she assumed my inflammation markers were always elevated because I don't recall my ESR and CRP ever being normal except for after starting Actemra. Then I was told my ESR and CRP were no longer reliable because of being on Actemra (tocilizumab) especially if I develop an infection.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8003879/

I have had PMR for over a year. My markers are high but still negative. I have experienced two relapses. My neck, shoulder,hips down to my knees sched badly. Going up to 25mg of Prednisone and slowing reducing now to 12 mg over one month I am able to return to playing pickleball 3X weekly with little pain. I am 74. My rummyatoligist also has me taking 500mg of sulfaSALAzine 2 tablets twice a day while tapering down on the Prednisone.

I would tend to agree with DadCue that diagnostic pain for PMR is bilateral but not always equal on both sides. I am 2.5 years into this journey and my shoulder pain goes all the way across and into my upper biceps but is sometimes worse on the right. I will get a feeling that there is sandpaper under my right shoulder blade and also stabbing pain which is worse under the right shoulder blade. I think that the PMR has caused bursitis in several areas and that's on top of aftereffects of a very active lifestyle and physical jobs. I'm right-handed so my right side has more of the damage.

I’ve had PMR for almost 4 yrs now and have developed very painful right hip and knee pain but still have overall body pain too …ie neck shoulders and hands …started Actemra 4 months ago but can’t say it’s done anything …finally got into see Dr and he thought I might have necrotic joints from prednisone …had lots of X-rays a few days ago and are awaiting results …really hope that’s not what it is as he says would require hip replacement .

I had a hip replacement 4 years ago but I had bone on bone. It's great now. I have had PMR for a year and have experienced hip,neck and shoulder pain relapses. The hip pain is at the level I had before my hip surgery. I just increase my Prednisone from my current level of 5mg to 20 and then taper down slowly for a couple of months and the pain is gone immediately. Hope your X rays come out OK but if higher levels of Prednisone doesn't work then you probably need to replace the hip.

HI @floridabonnie I have been dealing with annoying pain deep in my thigh for almost 1 year , coinciding with my PMR journey. I'm sorry to hear about your shoulders !

You wrote: "Since then I have had numerous bouts of what I thought was a relapse, which turned out to be something else."

What was the "something else" - can you tell us ? Are you questioning that Dx vs. a relapse of PMR ? Can you be referred to physical medicine /a physiatrist for an examination ?

- here is a scary title - High-dose short-course oral corticosteroid protocol for treatment of primary frozen shoulder: a retrospective cohort study [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8267043/] when i was wondering about the bilateral pain in my shoulders, i asked my aqua- Physical therapist - if she had ever seen "frozen shoulder " - she replied that she had , and i did NOT have frozen shoulder "

here is another article about prednisone and cervical Radiculopathy
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3743318/.