The cause of FMS (Fibro Myalgia Syndrome) remains elusive. In fact some of the symptoms often overlap those of CFS (Chronic Fatigue Syndrome) which is why most experts treat the two syndromes as the same. It is believed that there are many triggering events that precipitate its onset. These triggering events probably don't cause FMS, but rather, they may awaken an underlying physiological abnormality that is already present. Certain aggravating factors in your case could be stress, depression, anxiety, over-exertion and like you mentioned change in weather conditions (hence the cyclic nature) may all contribute to symptom flare-ups.
I am not aware of your sleep pattern but with FMS often it is noticed that even if you do fall asleep easily, your brain activity remains in a state of wakefulness, which may be a cause for your chronic fatigue and may require treatment. I would recommend that you continue to visit the doctor that you were previously seeing. You could try alternate forms of treatment on his/her recommendation, such as accurses, yoga, massage therapy etc. as well as a gentle exercise program along with medication. You could also consider consulting a psychologist/psychotherapist for stress management and treatment for depression.