What would cause a sudden srapid hearbeat and pvcs ?

Depends: Many factors. Stress, medication, sometimes no apparent reason.Electrolyte imbalance, dehydration, hypoglicemia and so on, just to name a few. Mostly it is benign, but a doctor visit should be warranted if this episodes have the tendency to repeat.

Abnormal rhythm: Abnormal heart rhythms can be due to ischemia (decreased oxygen to the heart muscle), changes in the heart muscle that change the electrical conduction in the heart, or congenital problems with normal electrical conduction in the heart. You should see your primary care doctor or a cardiologist as soon as possible.