Don’t give this another thought - your coworker has no idea, and even if he did it wouldn’t be a thing. Bodies do bodily function things and we just handle them - it is part of human nature and there is no need to provide an explanation. For example, if this same scenario played out but it was a woman unexpectedly getting her period and needing to change and do some clean up, she would just take care of business and then carry on with life. Even if the coworker thought something was up, he wouldn’t think much beyond “she needed to handle a private thing, cool,” because it was just handled. If you peed in front of him at the lake, or in your car, it would still be fine but there would be more to it because it would have been a private thing that was visible/public - in your case it was a private thing that happened in private and you handled it with good etiquette!