The flush can cause the gull and ducts to contract VERY hard. I too had the same symptoms under my rib cage as you. The pain has subsided and it took 2 weeks. I believe that the hard contractions of the gull can strain surrounding tissues and cause discomfort, tightness etc in the area.
It is also possible that your gull is having spasms after being stimulated by a the large dose of fat. The wall of the gull is a muscle and just like other muscles after exercise, muscles can become sore and get spasms. (especially if they are not used to it)
Let it rest and don't do anymore flushes for a while and the pain should subside. Its hard to just ignore the discomfort, but I relaized that not focusing on it helped. If I thought/worried about it the discomfort seemed to hang around. I'm fine now.