My 12 year old niece woke up with her heart beating fast,we gave her water and she started shaking;what could be the cause because she is a healthy child?

Check temperature: Provide fluid, a healthy meal/snack and monitor for fever. She should see her pediatrician for an evaluation. Infections may be associated with chills and rapid heart rate.

Possible fever: Please check her temperature; these symptoms go along with fever and are common and normal. As long as she is awake and alert, the shaking is probably from that. If she has a fever, you can treat with a fever reducer such as acetaminophen or ibuprofen and increase her fluids. If the symptoms persist, please see her doctor.