I had something similar happen to me about 6 years ago, and my doc at the time, couldnt find anything. He thought it may have been something 'viral' causing the symptoms. I would be sitting watching tv or relaxing and it would just happen, my heart would feel like it was speeding up and I would get flutters, and I would get a rush of panic as it would take me by surprise. It got so bad that I couldnt sleep most nights due to anxiety.
Go back to your doc, or ask to see a specialist. Request more tests. It may not be anything of concern. With me in the end they found I had 2 holes in my heart and I had a small stroke ( TIA ) late last year.