With Hydra, Winter was mostly on a liquid diet. On the run, he was eating far too little. Now, he has all the food his enhanced metabolism needs but his rr bowels aren't used to it.
+upset bowels of any sort,
+the slow, steady discomfort from being too full, especially in public
+Steve rubbing Winter's shit-filled gut to make him relax enough to actually excrete it
+the damage from repeated rapes to his rectum and anus means he either has problems keeping a larger amount of shit in or he feels pain crapping it out or (please) both
+extensive anal training to deal with the above problems, eg excercising his anal &pelvic floor muscles with a weighted plug or stretching his asshole with fingers and toys
+awkward arousal from the anal play and scat on both boys' part, culminating in
+actually sliding Steve's dick up Winter's asshole and shameless fucking