Relax! If you could play basket ball, your heart is strong enough!
At times, one is aware of heart beats - as you say, it may be anxiety
making one aware of normal heart.
Rarely, it could be a problem of hear nerves! there are ways (24 or 48 hours ambulatory monitoring - Holter) to record and analyse the beats. You may go for it - it will rule out and reassure! You are in teens and can expect a long future - that seems good!
Also an Echocardiography
is generaly done to rule out - eg Mitral Valve prolapse
Also, some basic blood tests like haemoglobin
and so on