What can cause irregular heartbeats in an otherwise healthy person?

Hi doctor,

I am 20 years old. Recently I lost a person from a heart attack, but he was not a relative. And then I started getting nervous about my heart. I went three times to the hospital five months and a month ago and checked my heart. The doctors did not find anything pathological and told me to calm down and not think about it. I have normal heartbeats of around 75 to 80 beats per minute when I am calm. I do not have any type of history of heart problems in my family. But since then, after three months, I have had short pain in my chest that lasts for two seconds, and I also had pain in my hands, feet, and head sometimes. But I am really nervous when the pain occurs. Yesterday I was in the stadium watching a football match, and before the match, I went to training and got really tired. During the match, I was nervous and had 120 beats per minute, but there was no pain and a little high heart pressure. I could not sleep that night. Now after two days, I am back to normal with no high pulse or pressure. Is the pain related to being nervous, or is it a heart problem? All heart checkups including an X-ray of the heart, blood tests, electrocardiograph, echocardiograph, and blood pressure tests were perfect. The doctors did not even tell me to do anything. They said to me that I was fine. I also had an increased heartbeat (about ten beats per min) when I stood up. But I do not drink water, hardly one liter of water per day. I do not eat well. Please help.