J.D.
M. - it sounds like a rice allergy (or maybe another food that she could be allergic to was cross contaminated with the rice - ie: made on the same production lines or facility). Keep a log of what he eats when, and reactions.
You could call the company and ask about shared lines or shared facilities with the cereal. It may give you some more info. You could try it again with a different brand, but i wouldn't. I'd assume it an allergy and go from there with strict rice avoidance, until you know more. Has he had rice in anything else? FYI - Earth's Best, which is organic, is one of the worst co's for cross contamination. We use Healthy Times.
How many times has he vomited from the rice? How long each time? How soon after eating it? Even though he tolerated it for a couple months, it could still be a food allergy. Sometimes it takes time to develop. It's possible a month ago it wasn't stomach flu, but a food allergy reaction.
What does your gut say? Have you told your ped? Look up food allergy info and see if that seems to fit.
Do a search for my posts to find a lot of info on food allergies, resources dr's etc. PM me if you want more info.
Dr. Kim at Children's Memorial Hospital (Chicago and Glenview annex locations) is amazing. 800-kids-doc
www.kidswithfoodallergies.org is the best resource for info, support etc.
www.foodallergy.org is premeire world wide org for FA"s w/ great info, tools etc.