What causes sinus tachycardia out of the blue with severe presyncope? not on any medication. no alcohol. no smoking. no drugs. just sitting down and relaxing. out of the blue heart pounding and feeling severely lightheaded?

How do you know it was sinus tachycardia and not paroxsysmal atrial fibrillation or other tachyarrhythmia without an EKG.or being on a heart monitor at the time of your symptoms? The associated presyncope makes me think it wasn't simple sinus tachycardia which usually is caused by factors like anxiety,fever,etc. Should you experience another such episode look into it with a cardiologist&monitor

Your assumption that it is sinus tachycardia is probably wrong. Description sounds like paroxysmal tachycardia. If this occurs with reasonable frequency, a Holter monitor may provide the answer. If infrequent, an electrophysiologist may be able to determine if you have inducible tachycardias. I would do it soon and know the answer witout having to guess.

this can also be a symptom of just syncope. Try and lay down, it it gets better then it is not due to your heart beating fast