My heart has been beating really fast off and on today, any ideas what it could be?

I guess is started last night when I was watching TV kinda late. My heart sped up and I felt like I had had a ton of caffeine (which I didn't). It also woke me up this AM and happened a couple times throughout the day. It eventually returns to normal. Besides that I have no other symptoms. I do have a VSD (hole in my heart, aka Ventricular Septal Defect) and have had it since birth. I didn't even know it was there until pregnancy where I had issues with PE's (clots in lungs) and they found it. I doubt very seriously if this is a PE...the symptoms are not similar at all. I have not taken any meds except my daily baby aspirin. I also have not been drinking or eating anything that might cause this. Any ideas on what could be happening?