Palpitation is a term used to describe an awareness of a change in the heart rhythm. It can feel like skipped beats (the heart seems to miss a beat and after a gap the next beat is felt as a heavy thud- almost like a flutter of the heart), or you may notice that the heart suddenly starts racing, even if you are doing nothing. It can last for anything from seconds to hours or even days.
Most people experience some palpitations during their lifetime. Sensations of missed beats are most likely to occur during illnesses such as colds or flu or if you are over-tired. They can occur if you are stressed or experiencing anxiety or if you have drunk too much coffee / tea / alcohol/or other stimulant drinks (e.g. Red Bull). They can be caused by taking certain medications like Sudafed or drugs like Ecstasy or Cocaine. A sensation of the heart racing for minutes or hours is reasonably common, especially in young adults, and may indicate an electrical short circuit in the heart. This is often associated with an uncomfortable awareness of your heart thumping in your chest and/or pounding in your neck. These types of palpitations often stop themselves, although you may sometimes need to consult your doctor. They are usually not serious or life-threatening. Any form of palpitations that cause you to feel dizzy, give you chest pain, make you very short of breath or cause you to faint / blackout could be serious and you should seek medical advice.