I know just how you feel. Right before 6th grade Jess had an EKG because the medications she was one made it necessary. We got the abnormal call and then were called in and told it was long Q-T syndrome and she could not start school or do anything out of our line of sight until she saw the cardio - and it would be four months before the cardio could see her (only pediatrician cardio practice in our state) so they thought we should look into a home defibrillator kit, though ins would not pay for one until she saw the cardio but it might be too late because she could just drop dead suddenly for no reason but getting excited over something on tv. This was my kid who played soccer spring and fall, and was super active.
Turns out she had NO problems. They did the EKG wrong because they put the leads in the places they put them on adults, not adjusting because she is a kid. We did push and insist that the cardio see us sooner and boy was I frosted at the hospital and pediatrician office for all that terror we all went through.
So it is ENTIRELY possible that this is absolutely NOTHING. Don't get too scared - we were even ordered a wheelchair as J was not supposed to even walk aroudn the house - how stupid is that? Just push them to get you in to the doctor in the next few days and don't worry too much. The cardio told us that MOST, as in over 95%, of the reports they get of kids with abnormal readings or other cardio issues are badly done EKGs rather than problems with the child. over 95% was the number the cardio used, not one I made up.
And if there is a problem? Well, mostly they can fix them with medication, diet and exercise. So it will be okay either way.