Sometime my heart beats really fast and suddenly beats became normal can u tell me the reason and also at that time i have problem in breathing ?

Panic: Sounds like panic attacks at your age, but to be extra safe, have a cardiologist check your EKG, a heart tracing done in the office. It could be an abnormal electrical activity in the heart.

Palpitations: Fast heart beats could be due to disturbed cardiac rhythm ( called arhythmia) or anxiety (sinus tachycardia. AS you are also having breathing issues , this could be due to serious problem or just kind of panic attack. The best advice you can get is from a cardiologist for now.

Tachycardia: you could be having SVT which is a form of tachycardia which can be intermittent. You need to see your doctor for evaluation.