I’ve experienced every thing you can have go wrong with your heart. I’ve had tachycardia, bradycardia, A-fib and 3 heart attacks. Yes, the tachycardia is causing your lightheaded feeling. Tachycardia means one of the heart chambers is beating way to fast, in my case 197 beats per minute. My right Ventricle is the problem area and is called Pulseless Ventricular Tachycardia, because your heart is beating so quickly, you cannot feel a pulse. Since one chamber is out of sync with the Atrial Chamber above, it tries to pump blood before the chamber is full of blood. This is what’s making you lightheaded, not enough blood is reaching the brain, so you get dizzy and my even pass out. I highly recommend you see a Cardiologist because you are causing your body harm since you’re not giving your brain and body enough oxygenated blood, which could lead to other health issues. Good luck.