What might cause heart palpitations?

Palpitations. These are caused by many things. Stress, excitement, irritants both physical and chemical, pressure like a hiatal hernia or esophageal push or pull, and primary heart things, the causes are very many in number.
Many... This should be evaluated by holter monitoring for frequent symptoms or an event monitor for infrequent symptoms. These are often benign premature beats but further evaluation should be undertaken. Consult with your physician.