Well, I do get the nervous butterfly-filled stomach and don't feel like eating, but I don't feel like taking a crap.

I do have IBS and I have medication for it, but it's not a serotonin kind. My intestines basically spasm and the spasms make me want to take a crap. So, it's not really a serotonin thing for me... if that makes sense. I take an antispasmodic PRN.

But I wouldn't say your theory is wrong. It's entirely possible that everyone has a "limit" on how much serotonin they can have before they feel like taking a crap. So, maybe your serotonin levels tend to exceed that "limit" and that's what causes you to crap... and maybe my serotonin levels don't exceed that limit.