Supraventricular tachycardia (SVT) is a fast abnormal heart rhythm that occurs because there is more than the typical one and only connection between the upper and lower chambers of the heart. Electricity can go down one pathway and back up the other one, and continue around in a circle very quickly. About 2/3 of babies with SVT have what we think is spontaneous resolution by about age 6-12 months. However, 1/3 of them can "return" later in childhood, adolescence, or adulthood. It isn't that it went away, per se, but that the heart rate slowed down enough as the child grew older so that an early beat could enter at just the right time to initiate the abnormal electrical cycle.
The good thing is that she is now verbal (as opposed to when she was a baby), so she can tell you if her heart is beating very fast. If that is what is occurring, she should be seen by a pediatric cardiologist for further evaluation. However, I would also make sure that she is having 32-48 ounces of fluid (water/milk), a salty snack (pretzels, saltines, pickles), not skipping meals, and eliminating caffeine on a daily basis, as dehydration in adolescents is common and cause a feeling of "faintness."