It sounds like you're having palpitations. I get them fairly regularly, usually when I first wake up in the morning, or when I just get out of a hot shower. They're really odd and a bit freaky. I've been to the doctor several times, and we've determined that they aren't dangerous or physical, but mental.
My palpitations are most likely caused by anxiety
. I'm not stressed out, or anything, I'm just a highly strung person in general. That might be what you're experiencing too?
If it doesn't happen often, don't worry too much about it. If it happens again, or starts happening regularly, you might want to visit your doctor just to make sure that nothing serious is going on.