You didn't mention how long the fast rate lasts. It sounds much like PSVT. What has worked for me for years is called the valsalva maneuver. I take a breath, hold it and bear down like having a BM. Hold for about 10 seconds and then release slowly. That often kicks the rhythm back to a normal rate. Some folks splash their face with icy cold water.
I get annoyed with doctors that don't take the time to explain what's going on with your health. Even if it's not life threatening (PSVT is not), it's good to have the information. Can you call and ask your doctor or the nurse to check your file, see what's been noted about your heart rhythm? There's no excuse to just say "don't worry about it, take this pill" and then walk out the door.
Hi, sorry to hear the BB didn't work out for you. You could try a calcium channell blocker now, like verapamil. CCBs do not block adreneline like BBs, they have similar a effect that BBs do but in a much different way, so the side effects are different and usually better tolerated.
If you are anxious at all, should address that too. You can take meds or look for other coping strategies.