After running, i have trouble hearing my heartbeat in my ears afterwards, why?

Pulsatile tinnitis: What you are probabaly experiencing is an increase in blood flow of the vessels around the ear that can occur during exercise or anemia. It can also occur when the vessels narrow or harden near the ear and the surrounding blood flow is turbulent. At your age, and because it occured afer exercise, the cause is likely benign. However, there are serious causes associated with this so see your doctor!