Coffee has a laxative effect on about
30% of people - it stimulates the distal colon (basically the end of your colon, so probably the rectum or sigmoid). It's not the caffeine. They don't know what it is, 'cos apparently coffee has 1000 different compounds. I'm not somebody who's affected, so I can drink even black coffee (slowly though, as it is kind of hard on the stomach).
www.medicaldaily.com/pulse/coffees-laxative-effect-why-coffee-makes-us-poop-energy-drinks-and-soda-dont-347060@topic creator - I think it's more likely that those drinks are stimulating peristalsis (movement of the intestines), thus triggering the need to go. Perhaps your second surgery has made your bowels more sensitive - it happens. It takes about
a year for the bowels to fully adapt to their new plumbing after any major bowel surgery, so give it more time and it may gradually improve.