"Anemia can lead to a rapid or irregular heartbeat (arrhythmia). When you're anemic your heart must pump more blood to make up for the lack of oxygen in the blood." https://www.mayoclinic.org/diseases-conditions/anemia/symptoms-causes/syc-20351360
You are getting iron infusions for anemia, correct? I am surprised that the doc wouldn't give it to you because you obviously need it.
I doubt that the rapid heartbeat is coming from the pred since you are only on 5mg.
You need to talk to a doctor about