I don't want to alarm you, but if this persists get to the ER. I had similar symptoms in the eighth month of my last two pregnancies, and each time I had gone in to Atrial Fibrilation. I would feel alright if sitting or lying down, and could hold a conversation with no problem, but the minute I got up to move around, it was as if I were carrying a huge weight, and I'd get very winded and feel very faint. Check your heart rate. Mine got up to 190 in a stationary state. If your heart rate is very high I would go to the ER. If it is A-Fib, then they will get your heart back into it's proper rhythm with medicine. My cardiologist felt that the stress of pregnancy put me into A-FIb and wouldn't expect me to go into it again unless I were to get pregnant again. It can be a little scary, but you'll be fine as long as you get treated. Good luck!