Is it normal that my 12 year old girl sometimes get one strong heartbeat while resting? it last maybe a second or 2 and then continue normal... no other symptoms?

A "strong heartbeat" in a "12 yo girl while resting" probably indicates a "pre-mature ventricular contraction" (PVC), or a "skipped beat" followed by a contraction with extra returned blood in the right/left ventricals secondary to the "skipped beat". Both phenomena are common, but see a cardiologist for a more definitive diagnosis.

Agree with Dr. Berryman. A premature beat is followed by increased filling and more complete emptying in the beat that follows. There is therefore an awareness of a “stronger” beat this is not indicative of abnormality. If frequent and the cause of anxiety, an examination may be warranted for reassurance of patient and family. I wish you good luck.