What Causes Fainting In A Woman Who Is Not Depressed And Physically Normal?

my wife came into hospital she has been passing out she falls to the floor she cant see but can hear she cant do anything for about two minutes then I help her up onto a chair some times she is thirsty she has had heart checks mmr scans they say theres nothing wrong physically. she is not depressed she is quite happy she takes thyroxscene tablets and has been low on iron she also takes iron supplement tablets.. two year ago she had a gastric bypass and lost ten stone she has now signed herself out of hospital as the neuro surgeon has said he thinks theres nothing wrong with her but there is some thing badly wrong she is passing out at least four times a day please help