I've basically only watched one video on this (of someone who's supposedly medically trained, has fibro themselves and published a book about fibro), so you know, don't think I'm an expert.

But well, according to this video, one common cause for pain in people with fibro is muscle tension. She said something like, what's normally considered dangerous levels of muscle tension, where you'd actively medicate people in a hospital, that's normal levels for fibros.

Obviously, you won't get muscle tension in an exam, except maybe in your writing hand, because you're not really using your muscles and exams tend to be short enough anyways.

Another suspected cause is that during fight-or-flight, your body releases testosterone, which inhibits, I believe, oxytocin production, which means your body slows down long-term regenerative processes. So, quickly closing up a bleeding wound is on schedule, but making sure your joints are regenerated before the next fight-or-flight situation, that's lower priority while you're supposedly still in a fight-or-flight situation.