When seated, i experience persistent lightheadedness, and, rarely, 2-3 second episodes of feeling faint. upon standing, this completely goes away. i can also recreate this feeling sometimes by tilting my head side to side. what could this be?

Usually it's the other way around when lightheadedness occurs. It's possible that upon sitting you are temporarily hindering blood from travelling up to your head by the abdomen compressing blood in the abdominal vein(inferior vena cava). Another possibility is stimulation of the vagus nerve on sitting down causing the heart rate to temporarily slow down. Tilting your head usually produces vertigo