Do you know what your iron levels are like? I ask because with my last pregnancy and this one too, I get similar symptoms; heart will start racing, I'll feel weak, out of breath (like my chest is being squashed) and dizzy. My blood pressure has always been pretty normal, so my midwife thinks its because of my low iron levels. When I take my supplement I'm much, much better. Not sure if that could be related or not for you though.