I had a similar experience recently. I was on MX for 18 months with tiredness 1 day a week as the only side effect. Since Jan this year, I ended up in hospital on several occasions with fever, increase in muscle pain, inflammation in spleen and pancreas and eventually severe anemia. All possible side effects of MX but not identified at the time of hospital admissions. No infection and all other possibilities ruled out, so MX was stopped. That was 10 weeks ago, and after a recovery period things have improved significantly. Although still taking too many steroids, bloods and inflammation are back to being normal for someone with PMR.