What Causes Feeling Of Passing Out Post Blood Transfusion?

After my mom had a blood transfusion which was a week before xmas she feels like she is going to pass out off and on through the day.I took her to emergency about a week ago and they kept her for 3 days and did many tests and found nothing.She is 90 years old.We will be going next Monday to her internest.I feel it has something to do with a reaction to the transfusion.Not sure what to do next.Hate to see her feel like this daily.