I am sorry to hear of your symptoms.
Fainting as you have found out by now has many causes though the vast majority are benign. But, it appears that you have already had a thorough clinical examination and investigations as you list MRI and heart screening amongst them.
However it would be useful to know whether the MRI was of the heart or brain and whether ECG, Echocardiography, Holter, exercise stress test and Tilt table test were part of the tests.
Your family history is also important.
If these tests were indeed perfomed and were normal, the most likely cause is Vaso-vagal syncope or VVS, which usually occurs in young adults and has "autonomic" features such as vomiting, sweating and greying of vision etc.
You appear to have had some warning and took the appropriate corrective steps such as drinking water and sweets. I would suggest that you may want to revisit the physician who performed your health screening earlier.
If the episodes are fairly frequent and disrupting your quality of life medication maybe required occasionally.
Have a look at this link for further info.
Hope this helps!
Regards
Dr Nandakumar
