I Collapsed Due To Iron Deficiency Anemia & Had 3

I collapsed due to iron deficiency anemia & had 3 units of blood transfusion. I also started taking iron tablets as well as vitamin d . All this happened in mid January. I have been having random heart palpitations for the past 2 weeks almost on daily basis. I have had an ecg 3x but it came clear. Really frustrated. I feel chest pains and so weak after an episode that normally lasts abt 30mins. What could this be?