Palpitation is described as feeling of heart beat. It can be fast ,prolonged and sustained. Its important to know and what triggers them, how long they last, how they are terminated, and how frequent in a day or a week.
Short i.e. less then 1 minute and self terminating should not create alarm or panic but they need to be investigated.
There are various causes of palpitation i.e. metabolic disorders, coronary artery disease, infection of any part of the body,drugs,electrolyte disturbance,arrthymias,and some can be benign.
You are already having an heart ultrasound, what I further suggest to have some blood test like full blood count, thyroid function test calcium,magnesium,urea ,electrolytes, urine DR.

Hi friend,
Welcome to Health Care Magic

She needs more investigations...
You have not stated your personal habits, family history or co-morbidities, which could have an influence on the diagnosis and outcome...
- HOLTER (24 to 48 hour ambulatory monitoring) is done to analyse and plan for further management, if a routine ECG does not show. It may also detect silent (no symptom) ischemia.
- TMT (Treadmill exercise ECG) is done to exclude Ischemia (reduced blood flow- Coronary Artery Disease).
- LABORATORY work-up is routine - like blood counts, sugar, urea, electrolytes (potassium in particular), thyroid (over-active) and so on. Your Internist shall suggest based on the situation.
If there are no clues and if the problem is still bothering to the extent of interfering with life style, she must go for Electro Physiological Studies (EPS)

Take care
Wishing speedy recovery
God bless
Good luck