What could be the reason for tachycardia and shortness of breath?

Hi doctor,

I am 23 years old. My height is 163 centimetres, and I weigh 140 pounds. I eat relatively healthy, and have a healthy smoothie every day, and I do not enjoy sugar at all.

I have had tachycardia four times since my junior year of high school. The third time it happened to me, I was playing racquetball and was rushed to the hospital. They gave me some medicine to restart my heart twice, but it did not work. Eventually, my heart rate went down on its own. After this happened, my heart would often skip beats. I went to a cardiologist for this; he said I was fine, and my heart was just filling with more blood than usual. That is why it felt like my heart was skipping a beat. He thought I had tachycardia episodes because I had low potassium. This has deterred me from exercising for the most part because of the fear of something bad happening to my heart. I actually had a panic attack for the first time after working out with some friends a couple of weeks ago.

Now, to get to the symptoms I am having currently, I was mowing the lawn, and I felt short of breath, and my heart was racing. I had to take breaks often just to catch my breath. After I have finished mowing, I felt really fatigued. I can hardly stand it. I am now lying down and feeling a bit better. I just bent down to pick up my dog's water bowl, and my heart skipped a couple of beats.

Thank you.