My heart suddenly starts to beat faster but then it goes back to normal with no more symptoms?

Avoid stimulants. Avoid stimulants such as caffeine, alcohol nicotine and decogestants ( as well as illicit drugs) and stress. If it still happens see a doctor.
Hard to say. This symptom could be related to a rhythm problem in the heart. In general if someone is at rest, and suddenly notes a fast heart beat, which then either gradually or suddenly returns back to normal, this is possibly related to a heart rhythm problem. If this is happening frequently, wearing a portable event monitor for 30 days can capture the rhythm and help make a diagnosis.