Palpitations on mild exertion.

27years male otherwise healthy 2months back in gym while doing treadmill I felt my heart is out of my control and as if I was fainting.. immediately stopped treadmill and  went to ER room done my ecg which came as tachycardia with heart rate of 105..the next day I went for ecg echo and tmt which came as normal..from then I often have palpitations on mild exertion and some times even at moving my posture.. I feel light headed and chest tightness..I feel scared even to climb stairs now...I was under severe stress since then..my friends tell me these r panic acctacks..when such palpitations r felt classic symptoms of dizziness and shortness of breath and sweating of palms and soles r seen..today I have pain in my left shoulder blade..is it due to anxiety or I have some arrythmeias? Please help..