Atrial fibrillation or AFib is a form of heart condition that prompts the upper chambers within the heart to act abnormally. Small vibrations prevent the heart from pumping as it should. In a typical person, blood moves from an atrium to the lower chamber of the heart, where it can be moved through the rest of the body. However, if the atrium is quivering instead of pumping, you might feel as though your heart has skipped a beat. You may feel nauseous, weak, or even short of breath.