Let me at the outset tell you that it is unlikely to be serious. Still you need to be evaluated, just for the sake of caution.
You should be evaluated for the following which are the commonest cause for sudden fainting or syncope;
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 and is a major cause for syncope. 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).
3) carotid hypersensitivity syndrome is an exaggerated response to carotid sinus stimulation. In some studies almost 50% cognitively normal people who had experienced non accidental fall or fainiting were diagnosed with carotid hypersensitivity.
4) orthostatic hypotension is due to failure of autonomic nervous system and related heart regulatory mechanisms.
Some of the following investigations may also be advised by your doctor;
1) MRI with gadolinium contrast
2) Magnetic resonance angiography (MRA)
3) Tilt table test
4) Carotid sinus massage
5) Electroencephalography (EEG)
6) Stress cardiac test
7) Total creatine kinase (CK) and heart enzymes; done immediately after the fainting episode and would tell about the abnormal brain or heart activity which caused the fainting.
8) Following also may be done after the fainting episode; blood electrolytes and blood glucose.
These should be able to zero down the cause.
It is privilege assisting you.
Please let me know if you have further queries or unanswered questions.
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Wishing you all the very best in life.