There's very little in cupcakes or baked cakes that could give you food poisoning - in fact, the only thing that I could think of would be in the fat (butter or shortening) had been rancid, but you would have tasted that straight away and left it, because it would have been rank.
I'd say probably coincidence, unless it was just a bit too rich for you.
I recently ordered a birthday cake from Magnolia for my DS and had no problem and didn't hear from any of the other mums either. Also bought cupcakes and desserts. They are delicious.
The first time I thought it was a coincidence but a friend and I each had a cupcake from Magnolias and for the rest of the night we had extremely sore stomachs. Didn't know what the problem was but after eating another one yesterday (months later) and having the same stomach cramps as before, I now know it was the cupcakes. Wondering what the problem could be--never have any problem like this before. Also wondering if anyone else experienced the same thing after eating from this bakery??