What causes heart palpitations?

Hi doctor,

I am an active 30-year-old female with health and generalized anxiety. For the past 4 days, I have been experiencing heart palpitations. Sometimes it feels like a small flutter, as if my heart misses a beat. Other times, it feels like an intense pounding that lasts for hours. I experienced these approximately 7 years ago when I was first diagnosed, but have not felt them in a long time so I am utterly terrified. For reference, I do not smoke or use drugs, and I also workout five to six times a week (cardio, running, and weight training). I went to see a walk-in clinic doctor who listened to my heart and said he thought it was all anxiety and suggested I take medication for my anxiety. I had chest x-ray, EKG and blood tests and everything was normal. Honestly, I am just terrified that I am about to have a heart attack every single time I feel these palpitations. Can these palpitations really be caused by anxiety? And should I be worried or just wait for them to pass? I ran yesterday and today in spite of them and did not feel any palpitations while I was exercising. I only seem to feel them at rest or when lying in bed. Please give me some peace of mind. I am going out of my mind with worry every time I feel these.