A positional change in the sensation of your heartbeat may simply be a change in perception. However, in some of us part of the upper chambers become sensitive to particular positions triggering rapid heart rates. There are several types of rapid heart rate, some of no concern and some that bear watching. For someone in the office, I would ask about other possible triggers like large meals, very hot or cold drinks or occasional symptoms while falling asleep. All may be clues to the type of rhythm. The evidence will come from a recording while it happens. Ask your doctor to arrange a recording. Then, engage in the activities that cause symptoms. This way you both know what it is and if anything needs to be done.