Your QT time will change all the time, it depends on a lot of things; heart rate, what you eat, activity, stress, medications, etc. One EKG showing borderline prolonged QT is NOT diagnostic of long QT. Also, a cardiologist must interpret your EKG, as the machines often are wrong, and prolonged QT will also usually manifest with specific changes in the T wave too.
There are two variants of long QT.