Can sinus tachycardia cause lightheadedness? i just found out on the 15th. it's only been going on for at least a week and only when i go up stairs.

The lightheadedness: you are experiencing is probably due to low blood pressure and the sinus tachycardia is a physiological response to this. Not the other way around. Sinus tachycardia does not cause lightheadedness. These findings may be suggestive of autonomic dysfunction or a disorder called POTS (positional orthostatic tachycardia syndrome.) Ask your health care provider if you are being evaluated for this.