ECG, 2DEcho, NT Pro BNP, Chest Xray Nrml

Hi Doctor, I am a 32 year old women, 5.2 ft, 72 kg. For past 2 months I have been a dull pain in my chest and left hand up till my fingers. The pain isn't constant, at times it's for few minutes or hours. And there is discomfort in chest at times feels like a burp is stuck or a pinching like pain. There isn't any pattern that I have observed. Sometimes I also feel LITTLE better after passing air or burping but not always. Frequency of burping is gone high in last two months. After consulting a doctor I have done my ECG, 2DEcho, NT Pro BNP and Chest Xray which is all normal. My blood sugar is normal but my B12 is low. Doctor has suggested me to do a stress test and asked me to take Ecosprin AV 75 and Aisa. What could be the possible reason for this discomfort?