Hi Purplegirl (Ps I love purple too!). My heart does not race constantly (and if yours is racing all the time, I would definitely get to a doctor), but I have definitely had times where it is going crazy fast. I measured my pulse at 130 one night, when I couldn't sleep because I was gasping just I'd just run up a flight of stairs. And it got to 180 once after going up a single flight of stairs! And I used to be in shape! I swear!
My ND believes it is from Babesia in my case. But there are other things too, like autonomic dysfunctions, that could mean that your heart is getting the wrong signals, and beating fast because it THINKS it has to, when it really doesn't. And obviously anxiety can cause it, but typically if that is the cause you have other symptoms of a panic attack, and it doesn't keep going indefinitely.
This is something you should definitely check out with a doctor