I just saw a 'Dr. Oz says'.... regarding eating a lot of potatoes, green peas, pastas and other starchy foods. He said they can lead to "Fecaliths: A hard stony mass of feces in the intestinal tract. A fecalith can obstruct the appendix, leading to appendicitis. Fecaliths can also obstruct diverticuli. Also known as coprolith and stercolith."
His remark was that the harder stool can move VERY slowly through the intestine, spending a lot of time on the right side and cause either appendicitis or even diverticulitis. Any starchy food will move slowly. The colon will removed too much moisture because of the slow movement.
Since this was discussed as being right in the area you're describing, it made me wonder if it might be your problem. I looked up some Google images and saw some surgical photos showing a sausage looking thing that was removed from the area of the cecum. These were of people who became impacted due to NO bowel movement and had to have surgery to remove the feces.
How about using a stool softener or even a laxative this weekend and then changing your diet for awhile to avoid anything starchy to see if that gets rid of the lump?