This happens to me a lot and can be scary and uncomfortable! My doctor says it's totally normal because of all the extra blood in your body. Drink lots of water and change positions when that happens. It helps me! Good luck.
Take deap breaths and a big glass of water. This happens to me often and I always get my husband and ask him to talk to me about something to help take my mind off it. And yes sitting up will help. I find that this feeling gives me some anxiety which makes it worse, but of you feel like it's serious and if it would make you feel more relaxed definitely go see a doctor who will listen to your heart and give you the re-assurance you need :-)