These are called palpatations. There are several reasons that can cause you to have these sensations. Stress, age, weight, diet, drug use, faulty heart valve- any number of possibilities. A doctor is the only one who could determine the cause of your symptoms.
This is a cardinal sympton of Premature Ventricular Contractions. (PVC's) for short. The atria fire normally but the ventricles misfire or fire too often giving you the sensation of fluttering. This is quite normal in the average adult.