What Causes Persistent Low Grade Fever With Body Aches?

I am female, 44 years of age, 160 cm in height, 68-69 kgs in weight and have a medical history of PCOD, small fibroids (I had a larger uterine polyp removed in Dec 2003), highly irregular periods - under control since 2004 with Dianne 35), leucorrhoea, and was diagnosed in Feb 2010 with hypothyroidism and a VitD3 deficiency. I am also prone to insomnia, and to fibromyalgia - specifically in the neck, shoulder, upper back regions and I periodically undergo myotherapy for this which works like a charm. My current health query related to a strange low grade fever I ve been running for months - since September 2010 actually - which typically hovers between 99.4 and 100F with the only symptoms being chronic exhaustion and body ache. The maximum I ve noticed it going up to is 100.3 - except for the time when the symptoms turned flu like and the fever went up to 101.8. I ve had all sorts of tests done between September and December last year but all my readings were normal (even ESR was