Hi,
For a more authoritative explanation of the possible causes, please see the Doctor's response to a question similar to your at the following link:
http://www.medhelp.org/forums/cardio/messages/31244.html
The link is to another posting on the Med Help Forums.
Lastly, I would consider seeing your Family Doctor and explaining your symptoms and concerns. An in-office EKG and 24 Hour Holter Monitor may shed some light on your symptoms.
Be well.