Irregular heartbeat

So, I went to my OB appointment a week ago and as she was checking my babie's heartbeat she noticed it was irregular heartbeat (I noticed too) so right away she scheduled an ultrasound to check the babies heart and told me not to worry. I didn't hit me until I left the appointment and just broke down crying in my car. I'm so worried. My ultrasound isn't not until Wednesday. Has anyone else experienced anything like this? I'm so scared.