Is your stool of the same consistency as before ? Are you holding back longer than before ? I have the same kind of pain when I have to hold back gas/stool for longer hours of the day and cant pass gas while sitting. I thought of it as pouchitis but my surgeon rejected that given I have the same consistency and no leakage and same number of bms . He said its the excess pressure on the butt causing the pain on spinchter muscle and is carried all the way to back and legs. Gas is pretty devious you never know what part might get affected. He asked me to change my diet , to lower the protein intake and lesser gassy foods. Its too soon to say but its kind of working.