Your description of physical distress sounds a lot like mine. I was diagnosed with PMR after 4 months of pain, stiffness and fatigue (PMR can give you anemia). I too was an active person and initially thought my pain was due to strenuous exercise, but not the case. It got worse and finally moved to my shoulders and upper arms; then I got a diagnosis.
I too felt worse after resting, couldn't get comfortable no matter what I did and went up and down stairs with great difficulty, one step at a time. Odd bodily things came with it: for a while I had to pee every hour on the hour at night, struggling to get out of bed and use a female urinal because I couldn't make it to the bathroom. The fatigue was extreme. Bowels were loose and barely made it. It was all from PMR and I doubt if the docs could even say why.
I am on 10 mg of prednisone now and feel 95% like my old self. It keeps improving and I am hopeful of getting over this thing!
If you do go on prednisone keep an eye on your bones. If you are anywhere near osteoporosis talk to your doc about bone drugs. Best of luck and good health wishes to you.