According to Mr. bsb21's profile he is Marriott Gold, however, lying about being full to avoid a late checkout seems to be the norm IME, see:

http://www.flyertalk.com/forum/marri...-question.html

I think you did very well by getting the points and the meal removed. As Mr. cyberdad states and my own experience, stomach distress can come from anywhere and strike anytime so its hard to point at one source with real certainty.

I would be livid that you were indeed having a medical indecent where a few extra hours of comfort could have helped out the situation and you were denied and lied to. To me, that seems like the bigger sin that warrants follow-up and explanation. I can see a GM not wanting to admit in writing that they caused food poisoning given the uncertainty of the cause.