The colonic stirred up and hopefully emptied a lot of waste from your colon, so even a day after, things are happening in there. If the colonic thoroughly cleansed the descending colon and part of the transverse colon, then the next day your natural peristalsis would be moving waste matter back into those empty parts, which may include some old formerly impacted material which is decaying and producing gas.
Look at this as a good sign.
Also, during the colonic you don't need to try and hold anything. Just let the water and the therapist do the work, with alternating inflow and outflow cycles and gentle abdominal massage to help break up pockets of hardened waste and stimulate release.