With information you have given you must have been detected to have some kind of Valvular thickening most commonly mitral valve which occurs due to Rheumatic involvement and to prevent streptococcal infection
this penicillin is given as prophylaxis Penidura every 28 days it should be continued up to age of 35 to 40 years of age as advised by your cardiologist.
Please visit Cardiologist he will evaluate by a 2 D Echocardiography
to see if there is any progression of disease and suggest proper treatment.
If this progresses to stenosis there are many complications so please do not ignore your health.