Hello, my name is Samantha and I am 20 years old, 5'8", 300 lbs, 7 months pregnant, and only medical history I have is being overweight and I had surgery when I was 5 because my umbilical cord never disolved and it was removed. Other then that I am healthy. Yesterday 9-11-11 around 5 pm I was lying in bed and it felt like my heart was pumping in my throat and it got really hard to breath and I was lightheaded. Since then I have been having it on and off. Its irregular and I never know when to expect it. What could this be?