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At this young age, congenital heart disease
is the most likely cause.
Some heart murmurs
can and do disappear.
Probably, a small hole was there and got closed.
The best option is to see a cardiologist - have physical examination / ECG / ECHOcardiogram
& basic blood tests.
It is better to have it examined than go on guessing. You are young and have many happy years ahead. Get tested / get reassured.
Wishing speedy recovery