Hi sjogrens disease causes generalised xerosis of skin which inturn causes genaralised darkness of skin.even doxycycline is a photosensitive drug which causes darkness of skin.i think there is no relation with fibromyelgia .
Go for thyroid
profile.serum ferritin..serum b12 and haemogram.
Apply sunscreen in the morning and moisteriser in the night.use supplementation of iron.folic acid
and b12 vitamin which cures the pigmentation part