Elevated heart rate?

I went to the doctor yesterday and she said my heart rate was pretty high. The whole time I've been pregnant I have had moments where it is elevated but it always goes away when I sit down and rest. But this week, I noticed it was really bad when I was getting ready, to the point where it felt like my head was throbbing. 
Today, I felt fine. I was up and around making lunch for my husband and I, and when I sat down and started eating, whithin five minutes my heart started going crazy and I got over heated and felt really light headed and faint.
Does anyone have any information or experiences regarding this?