Somethings wrong with me heart?

Ever since about 8th grade(will now be in 10th) sometimes i feel like my heart is off beat and painful. Yesturday I was in the pool swimming some laps with my friends. When I got done for just one beat I felt it like idk do something. and it felt like a "pop". Ive been to the doctor once specifically about heart discomfort. He listened for about 10 minutes and didn't hear one slip up or anything. Ive had two physicals and they have never heard anything wrong with it. But I feel like somethings wrong sometime. What is going on? Is this normal for it to just slip up every now and then? Oh and im a 15 yr old guy active cross country and track runner.