What causes low blood pressure?

My girlfriend suffers from bouts of breathing problems where sometimes she can't breathe for periods of up to 10 or 15 seconds almost like an asthma attack. I've put this down to low iron levels (already tested low) because when she takes an iron supplement every day her breathing slowly returns to normal. But when she has these attacks her blood pressure is always very low so I'm trying to understand what causes her blood pressure to fall.