unexplained chest pain

for the last 3 months i have been having chest pain and  what feels like pressure around my heart. it was intermittent but in the last couple of weeks its been almost constant. and its worse at night. i've been to the ER twice and they foun d nothing. I've been to a cardiologist and he did ekg,a sonogram of my heart, a sresstest,and made me wear an even monitor for two weeks. everything came back negative. but the pain radiates into my left shoulder and left side of me neck..sometimes i get naueaous..and the kicker is my cardiologist gave me some nitrates and they worked!! but they cant find anything wrong with my heart. sometimes i cant breathe and i feel like i'm smothering..a couple of times my left arm went numb. what is going on?? i'm only 19!!