Splenda has malitol and sucrolose in it - in larger quantities, they have been proven to cause bad stomach cramps and diarrhea.
On one of my cheat days about a year ago - I decided to eat a whole pint of the Atkins Pralines & Cream ice cream. The next day, I had stomach cramps & diarrhea so bad I couldn't leave the house. It was horrible. I haven't eaten any since.