it's supraventricular tachycardia basically, all it means is imagine the nerve which signals your heart to beat is a little circle, each time it pulses round, that is one beat. mine (or anyone else's with svt) has a web of extra nerves within this circle, which means it short circuits sometimes, creating a much smaller circle, which in turn creates a faster pulse, top speed is about 200 beats per minute.
it's not serious, but can make you feel dizzy, disorientated, lips go blueish, feel cold, etc during an attack, then feel very tired afterwards. mine took years to be diagnosed, i had regular blood tests, chest x-rays, but it was only when an attack was finally caught by an ecg, they could tell what it was straight away. it was much more frequent when i was younger, i don't have many problems with it now. if this is what your dd has, it is very manageable, and in rare circumstances where it does become a problem, nowadays, they can laser away the extra nerves, by keyhole surgery, going in through the groin, all done as a day patient!