Dear CAM,
It is difficult to say exactly what is happening here without more history. The two possible things that this could be are an arrhythmia, which is an abnormal fast heart rhythm, or sinus tachycardia, which is a fast normal heart rate. She may need to have a special monitor to diagnose this, or it may be able to be ascertained by history, exam, and ECG. To rule out sinus tachycardia, I would make sure that she is adequately hydrated, taking 32-48 ounces of fluid per day, not skipping meals, and not having any caffeine intake. This may help prior to your visit. If she passes out with this or has significant chest pain with this, she should be evaluated in the emergency department sooner than her outpatient visit,.
Sorry - forgot to add - she has no other symptoms at the time of these episodes, no dizziness, no shakiness, no nausea, nothing - just a racing heart and feeling like all the blood has gone to her head.