It is possible, Food Poisoning can occur from something that was not your last meal. Since both my spouse and I are sick it is something we both ate. I understand the ideology, but I would think in either transport or manufacturing the product could have been tainted.
Capt. von Richthofen is coming after us one at a time. Revenge is a dish best served cold!
My guess is its a mild form of E. Coli that made you sick. Frozen
pizza has meat and veggies, and there have been outbreaks
of E. Coli in the past related to pizza. Your back hurting is your
kidneys recovering from the infection. I'm not a doctor outside
of my home office...
I think at 400ºF for 20-25 minutes(typical frozen pizza cooking temp/time range) E. Coli would be wiped out. However, if you handled the pizza before the cooking process and didn't wash your hands prior to eating it, then there could be E. Coli on your hands and re-transmitted it to the pizza when you ate it.