Your symptoms of dizziness and vomiting suddenly with no cause or prior warning with sudden blackout and extreme sweating are quite significant and need pin pointing of the cause. So a thorough assessment is needed by various investigations and appropriate address by the following specialists;
2) Ear, nose and throat (ENT) specialist
Following causes can cause sudden blackout;
1) bradyarrhythmias; this is sudden slowing of the heart. it is mainly due to sinus and atrial node dysfunctions. Sick sinus syndrome is a prototype of these arrhythmias. 24 hour Holter monitoring, echocardiography etc are the investigations done for it.
2) neurocardiogenic mechanisms; this is fall of the blood pressure suddenly due to a sudden sympathetic failure (failure of the autonomic nervous system). It can be accompanied by flushing, nausea, and sweating.
3) carotid hypersensitivity syndrome is an exaggerated response to carotid sinus stimulation. In some studies almost 50% cognitively normal people who had experienced blackout were diagnosed with carotid hypersensitivity.
4) orthostatic hypotension is due to failure of autonomic nervous system and related heart regulatory mechanisms.
Internal ear disease vertigo is termed true vertigo in which one feels rotational movement of self or the surroundings. The false vertigo or nonvertigo has feeling of light-headedness, unsteadiness, motion intolerance, imbalance, etc. You need to consult first an ENT specialist to exclude the diseases of internal ear, if not yet done. They will perform some clinical tests (vestibular diagnostic tests) to establish the cause. Often, dizziness can be a multisensory disorder due to any combination of peripheral neuropathy, visual impairment, and musculoskeletal disease. Many patients who have thyroid dysfunction can present with dizziness as an initial complaint. In individuals who have migraine; dizziness and vertigo can occur as part of the aura or separately. Spells usually last approximately an hour but can last several hours or days in patients who have severe symptoms. Most patients who have migraines have a long history of recurring symptoms. So that too should be assessed as a cause since you have a history of migraine.
Since your MRI / CT scan has not revealed anything, your physician may consider MRA (magnetic resonance arteriography). That way any cause related to vascular disorder can be ruled out. MRI with gadolinium enhancement is particularly useful in detecting smaller intracanalicular tumors such as acoustic neuromas. It is also recommended for identifying sclerotic and demyelinating white matter lesions characteristic of multiple sclerosis. If you are on any medications, their side effects also should be ruled out for lightheadedness, as drugs are quite a common cause for this symptom. Dizziness may be provoked by only certain movements, such as standing up after lying down for at least 10 minutes in orthostatic hypotension, or may occur after vertical or oblique head movements, such as lying down, turning over in bed, or sitting up in benign paroxysmal positional vertigo (BPPV). The causes of dizziness according to the decreasing incidence are;
1) Benign paroxysmal positional vertigo (BPPV) 20%
2) Vestibular 15%
3) Migraine and motion sensitivity 15%
4) Anxiety and depression 10%
5) Meniere's disease 7%
6) Cerebellar disease 6%
7) Transient ischemic attacks 5%
8) Orthostatic hypotension 4%
So crux is further investigations and involvement of various specialists so a cause can be determined and a definite treatment can be instituted to abate the symptoms.
Please feel free for your follow up questions.