I am an 18 year old female. I am 5'3" and weigh about 102lbs. I have been experiencing a hard heartbeat while doing simple tasks or even resting. I have also noticed that my pulse is very visible in my neck, especially on the left side. On the left side where most of the pulsing is, my head hurts and i get pressure behind the ear and neck pain. I experience shortness of breath and pain in my rib cage and down my left arm. I had an EKG done and it came back clear, and a clear chest X-ray. Does anyone have an idea what could be wrong?