I just returned from a visit to the Mayo Clinic, where I was seen for many different health problems, one of them being extreme tachycardia. They discovered while I was there that I have what is known as a compressed artery, which leads to my heart rate changing as I breathe in and out. This is commonly seen in athletes and/or young women who tend to be rather slim. This could be what is happening to you. As far as I know, it's no big deal unless more than one artery is compressed, which leads to bigger problems. Otherwise it's nothing.