What Causes Severe Pain Under The Breast?

Hi, im 28 yrs old and have been suffering from left upper quadrant pain right under my breast for a month and a half. I went to the emergency room 2x for this. They believe it to be acid reflux. Ive been on nexium for this but the pain is still there and radiating to my right side and back.
Its worse at night. I have change my diet and still no relief. What can or should I do?