You were most likely drinking too much water during your fast.
Drinking more than 3 liters of water per day, while not eating anything, may not be a good idea.
Consuming excessive amount of water will result in washing too much minerals (sodium chloride mostly ) from your blood, and one of the symptoms of low sodium inside body is low blood pressure, fatigue and also fainting due to low blood pressure.
You fainted because of the sudden drop in blood pressure.
Below is a list of some common symptoms of low level of minerals in blood. These are just the most common symptoms, there are many other symptoms not as common.
fatigue or low energy
low blood pressure
muscle cramps or spasms
Beware, that long term Water Fasting may give natural weakness, fatigue and low energy in general, even if your sodium levels are ok. That is due to low levels of blood sugar.
Potassium ( the best source: fruits and vegetables )
Low potassium symptoms
Symptoms of low potassium are usually mild. At times the effects of low potassium can be vague. There may be more than one symptom involving the gastrointestinal (GI) tract, kidneys, muscles, heart, and nerves.
Weakness, tiredness, and cramping in arm or leg muscles, sometimes severe enough to cause inability to move arms or legs due to weakness (much like a paralysis)
Tingling or numbness
Nausea or vomiting
Abdominal cramping, bloating
Palpitations (feeling your heart beat irregularly)
Passing large amounts of urine or feeling very thirsty most of the time
Fainting due to low blood pressure
Abnormal psychological behaviour - depression, psychosis, delirium, confusion, or hallucinations.
Magnesium ( mineral water , it is also a natural part of unrefined Sea Salt , Epsom Salt bath , bathing in magnesium chloride )
The classic physical signs of low magnesium are:
Irritability and anxiety
Impaired memory and cognitive function
Anorexia or loss of appetite
Nausea and vomiting
Irregular or rapid heartbeat
Severe symptoms of hypocalcemia include:
confusion or memory loss.
numbness and tingling in the hands, feet, and face.
weak and brittle nails.
easy fracturing of the bones.