There are different reasons for a heart arrhythmia one is a heart beat goes to slow or fast and I have tachycardia where one of the valves tries to go faster than the others.
There are lots of causes. Structural defects in the heart, illness like an infection in the heart tissue, certain drugs can affect heart rhythm. Shock and distress can upset your heart rhythm, as can heavy drinking. My missed beats began after an operation - I took a long time to come round after the general anaesthetic and was later found to have 'multiple ectopics'.
If you feel you are having arrythmia, you should see a doctor. They will do tests like an ecg, an echicardiogram, or a stress test - this will give them a very good idea whether your problems are serious or not. Some arrythmias (like mine) do not even require treatment and even serious ones respond well to medicines like beta-blockers or surgical intervention.