High heart rate, shortness of breath

Has anyone else had problems with high heart rate and irregular palpitations during pregnancy? My rate runs around 120 bpm sitting down and when I am standing (and sometimes sitting) I have sudden bursts between 160-180 and get really dizzy and have trouble breathing. If I don't sit or lay down quickly I will start whiting out. I am on a beta blocker right now and it is reducing the frequency of the episodes but I am still so exhausted all day and can only stand for a couple minutes. Anyone have experience with this or know what causes it? I saw a cardiologist and my OB and got no answers but that they would put me on a beta blocker.