My 81 Year Old Mother Is In Congestive Heart Failure

My 81 year old mother is in Congestive Heart Failure and has Afib. Recently her Red Blood Count was low and needed a blood transfusion. That problem has been solved and her blood count is slowly rising. What I have notice the last few days is that her blood pressure is extremely low averaging 89/49. I have stopped her blood pressure medicine. With a little exercise her blood pressure is 115/60. Why is her blood pressure so low?