No one seems to know what triggers the onset of PMR. I know that mine had nothing to do with flu shot as it was 2 months after I got one that I noticed weakness and discomfort in the muscle of my right arm. Next came stiffness and pain in the shoulders and the hip problems..Most of this I figured was old age so I put up with it. Therapy followed for months and then my right hand became swollen and then finally I was so fatigued and began having difficulty getting up from a chair. and I knew I had to see a doctor and find out what was wrong. After blood tests I started taking 20 mgs of prednisone and have .been trying to get off of it ever since. I have lost a lot of hair and my skin has suffered from taking prednisone and I would like to stop taking it one of these days but am going slowly as the last time I tried a lot of my problems returned.