How can I fix this?

I don't know if it is relevant, but I have recently started drinking G-Fuel. I drank 1-2 cups a day, and rarely 3. As I was drinking G-Fuel, I was a little bit worried my heart rate would speed up, so I took deep breaths (breath in for 7 seconds, breath out for 7 seconds) every once in a while. Also, I have also recently been playing video games with a slanted back. I don't if that is relevant either, scene as everybody does so. Also, I am also 12 year old boy. So One time I am on the computer, have been sitting there for a while/a couple of minutes, and I start feeling a little tense and slightly light-headed. Then I feels 2 bumps of extremely, very slight pain in my heart. Then my heart starts racing. I get up, take deep breaths, and it soothes down after a couple of seconds. Then I eat and drink. After, I lay down. I feel slight pain in my chest as I am lying down. I get up. After a couple of seconds, it's completely gone. A couple of hours later, I sit down slightly. After a while/a couple of minutes of that, I feel, once again, very, extremely slight pain in my heart. Get up, stand, take deep breaths, completely gone. Now I am asking what is the problem and how can I fix it. I do not want to be worried, as this does not bother me too much and it is only extremely, very, slight pain. I think that I am healthy and in shape.