Heart palpitations

Hey all,
I am sure I am getting wound up about nothing but I am getting very bad heart palpitations. They are here nearly all day and at times can take my breath away. 
I have spoken to my GP, who says not to worry, however I am not convinced. If they are leaving me breathless what are they doing to baby?
I am 20 weeks, and have my scan on Friday, and will discuss with midwife then. 
I got them infrequently before pregnancy but since 18 weeks they have been constant. 
I am sure getting worked up isn't helping them!
I have cut out all caffeine over the course of my pregnancy so know it's not that. 
Just wondering if anyone else has felt the same or had the same issue and what caused it?
Really worried that I will drop dead suddenly!! My husband says not to be absurd and to trust the doctor (in an incredibly supportive way)