I believe it's bc of the increase in blood flow when ur Prego
I have had this issue the whole time but my baby is doing fine. Doctor says its normal
Make sure you are drinking a lot of water, a lot more than normal. It should help with the increase of blood that is being produced and the blood pressure issues. If you can, get your blood pressure checked out.
Recently I've noticed my heart racing while @rest also. I'm glad other expecting mommies are noticing this. My b/p is good so I'm not to worried. Definitely something I must look into. Ttys!