What Causes Constant Pain Under Right Rib Cage?

I have a constant pain under my right rib cage and sometimes it pulsates and flutters and feels bloated. I have had an ultra sound done and the results came back negative. My doctor has said that my intestines are swelling do to high stress. Is there any other problem that there could be that the ultra sound wasn't able to detect. I am so scared that I may have some type of cancer that was not seen by the ultra sound.