Tachycardia

When is it not normal in pregnancy? I have had several episodes, I've even blacked out a couple times. It's not minor either, we are talking heart rate over 200 for 30+ mins. I also have hyperthyroidism, but at the last blood draw, everything was normal. Had an ekg, it was normal. There are no rhyme or reason to the episodes. When they happen, I have a hard time breathing. I recently switched Dr's and my new OB is not concerned, but it is really scaring me. I am 32 weeks. Is there a point I should just say screw it and head to L&D/ER? Or is it really normal to be this bad?