You are not alone in not knowing if it is a flare vs. other sources of pain and discomfort. When you toss in symptoms of prednisone withdrawal and adrenal insufficiency it becomes impossible to separate one source of pain from another possibility. The difficulty is that prednisone relieves almost any pain that is caused by inflammation. Unfortunately, inflammation is a process that occurs naturally for many reasons. Even stress, both physical and emotional, in the absence of any injury or infection triggers an inflammatory process.

I like how people say you should "listen to your body" and take prednisone when you have pain. The problem I had was that my body would have pain but failed to inform me of the reason for the pain. When I took prednisone it usually relieved the pain so I assumed the pain was caused by PMR. Unfortunately, relying solely on pain levels can be misleading, as other conditions can mimic PMR pain and be relieved by prednisone.

It wasn't until I was able to control my PMR pain with a biologic medication that I could begin to make sense of what was PMR and what pain was caused by something else. The added bonus was that the biologic didn't suppress my adrenal function. That allowed my cortisol level to improve. Cortisol is the hormone that the body produces naturally. One of the main functions of cortisol is to regulate my inflammation so I didn't need to take prednisone anymore. Prednisone is more like taking a sledge hammer to inflammation because it is roughly 4 times more potent than cortisol.
https://www.healthdirect.gov.au/the-role-of-cortisol-in-the-body
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Biologics work differently than prednisone. The biologic I took acted more like a "surgical strike" that targeted a specific inflammatory protein called IL-6, rather than the broad, systemic suppression of inflammation caused by prednisone. Research has shown that excess IL-6 levels are implicated as the primary driver of PMR inflammation.
https://www.pmrandil6.com/il-6-and-pmr/