I have frequent which I take Imitrex (sumatriptan) for. I have yet to be told what could be causing them. I have fibromylagia and I understand that many people with this get frequent migraines. Is fibro the cause or could it be something else?

Associations. Not uncommon for migraine and fibromyalgia to be co-morbidities. Migraine is due to hypersensitivity of brain neurotransmitters to external and/or internal triggers. Imitrex (sumatriptan) is fine to stop headache at time of onset, but maybe you should seek a medication to prevent headaches and maybe help the fibromyalgia.
Ans. You should have a full evaluation for the etiology of your headaches. Even after this the exam and lab work and diagnostic tests could be normal and no etiology for the headaches .