Breathlessness, nausea, lightheadedness. breathlessness is worse while laying on left hand side. ecg's have come out good. any ideas?

Not heart: It doesn't sound like your heart so it's not surprising that your ECG is normal. Have you had a chest x-ray? Oxygen saturation? D-dimer (to rule out blood clot). I would be concerned about fluid in your lung. Do you have fever? Elevated white blood count? Cough? Phlegm? Wheezing? An ECG is not an adequate evaluation of your symptoms.