I think I've developed a sensitivity to cooked tomatoes. My symptoms were all digestive-rated -- nausea and diarrhea. There did not seem to be any histamine-related symptoms, like hives, sneezing, or difficulty breathing.

I googled food sensitivity and found some good sites that explained things.

Regarding the salads, you might try eliminating ingredients one at a time to see what causes the problem. Although since it's not a regular occurrence that might not work.

I used to have a similar problem with salads and it turned out the problem was the Newman's Own Italian dressing. I don't know exactly what ingredient caused the problem.