Iron deficiency can cause palpitatons, tachycardia, SOB, and dizziness. Did you see a doctor after you had the fainting episode? You should let your doctor know what's going on, even if you just leave word with his nurse by giving them a call. They may want to do a quick blood test or just try you on some vitamins....your symptoms are also signs of anxiety, so letting your doctor know everything that's going on is the best place to start. Best to you!
if they said you are low in iron then you are. the donor clinics do the iron test to see of you are ok to donate. you should see you dr to get that looked into. they may recommend you fix that with a diet that includes foods high in iron --- ie/ red meat, red kidney beans -- or they may suggest you supplement with iron pills, no higher than 18mg, or you may also have low blood sugar levels. see your dr, you could risk becoming anemic if you dont look after that issue