I feel week, have hard heart beats, nausea, and fatigue what do you think this may be?

Needs to be checked: We can not tell you what it may be as these symptoms can be due to many common conditions from anemia to some thing serious i will recommend you consult your doctor to find out what is going on.

POTS?: Teenage female with tachycardia, nauseousness, ; fatigue has non-specific symptoms. Thyroid problems, anemia, and autonomic symptoms might be explanatory. Hyperventilation is also of concern. Postural orthostatic tachycardia symptoms are a rare occurance. What is needed is a basic physical evaluation and laboratory studies as part of a differential diagnosis. Primary medical doctors can discern.