I had this happen when I was 8 months pregnant with my first child. The best thing to do is sit in a comfy chair and breathe in through your nose to a count of five and out through your mouth to a count of five. Do this until your heart rate calms down. Do not panic (that's what I did) and I ended up getting myself into a state. This type of thing can be caused by anxiety (mine was) but just doing the breathing exercise my GP gave me did the trick. If it carries on I'd go and see your doc x