How under the heaven did myself and my girlfriend eat the same food from the same source and I came down with food poisoning while she was bouncing around?

Lots of people, when they travel overseas, get food poisoning from eating food that the locals eat without issue. Since, as you say, you don't customarily eat junk food, and your girlfriend does, it could be that she grew a tolerance to the particular strain of relatively small number of bacteria that possibly exist in all these foods (including the ones you cook at home but which you learned to tolerate), whereas your system was taken by surprise by this new strain.