Some help would be awesome

So I went to the er today for stomach pain n nausea because it was just too much it brought me down to my knees it got better as time went on that might have been because they gave me morphine on top of the Tylenol I took at home. They didn't find anything wrong at the er so I got sent home n was told to just drink water n clear liquids. Well I didn't eat all day so I got hungry n ate a little bit not much of anything and the pain got worse but now its lessening what could be causing this pain any ideas would be helpful