I don't think anyone can be certain about the cause here. These situations tend to clear up by themselves without becoming serious, so you end up guessing at a trigger or just accepting the unknown. Stomach troubles from meals, tainted food, bugs from viruses or bacteria, or reactions to meds taken around procedures all occur pretty often. Loose bowels show up for most folks every so often, perhaps yearly or less, and it varies with how much contact there is with children carrying stomach bugs. Plenty of times no obvious reason exists. When things calm down in a few days as they usually do, without needing heavy medical checks, the only way to check if that substance played a role is pausing it and watching if symptoms return after starting again later. If the other medication is behind it, which does not seem very probable after tolerating it for a stretch without an error in amount, the issue might stick around awhile, though reactions from that type of drug are not always straightforward, so skipping the next dose until things ease could make sense.