I didn't get home until a few minutes ago to offer my two cents, but my bet is that Adrienne nailed it with classic sinus tachycardia. This is very common when someone is overworked, overstressed, or overtired, which is quite common around the holidays. Using caffeinated beverages to keep awake or alert can seal the deal further, as can be the case if you're just beginning to come down with something.
Most people get brief cases of this on occasion, as it is really just an extension of the increased heart rate that occurs from exercise. The only differentiation is whether the body slows it down again within an appropriate time span.
If she's not prone to arrhythmias and not in any other ways actively ill (fever, etc.), this is the most likely scenario. A doctor would be unable to find anything in that case. Wouldn't hurt to check with the General Practitioner on General Principle, though.
Best wishes,