Multiple causes. Most individuals have occasional irregular heart beats - this can be entirely benign or a serious cardiac condition - you need to have a thorough evaluation by a physician with an ECG to start with and further testing depending on your age, other medical conditions, medications, etc - important not to ignore this symptom - most of the time this condition is found to be benign but don't ignore it.
Many things can. An irregular heartbeat can have many causes from very benign ones to serious ones. It is important to review with your provider to be sure you are asymptomatic. Often an ekg is needed to be able to say more clearly what the irregularity is which can help decide its cause. Sme causes are from the heart itself, some can be meds, electrolyte issues, blood flow, caffeine/alcohol.