I'm not a doctor, but I believe this happens because the heart is in flight mode & the body restricts the amount of blood being flown around the body. By the sounds she has some type of tachycardia (but it could be other things) and should see a Doctor, especially if she experiences sharp chest pains.
The common causes are caffeine, stress, drugs, excess weight, stress & alcohol. However many other things can trigger an episode. Sometimes they arise for no reason, having laid benign since birth.
There is medication for this (calcium antagonists, beta blockers, etc) and their is an operation called an ablation depending if it is a tachycardia & what type it its. There are, of course, many alternative methods that can be tried- many with alot of success.
It's also good to look at if anything has changed in her diet, lifestyle and environment that many be factors. But again, this is only my opinion if it is a type of tachycardia. But best bet is to see a doc.
This forum is not monitored by doctors. Responses come from members like me who are just patients and who thus have experience with heart rhythm problems.
A sudden fast hear rate in the absence of any stimulus, something exciting or frightening, for example, should be check by a medical doctor who can see and talk with the patient.