Perhaps look into your dressing. I for example, am sensitive to raw eggs. It came on suddenly a few years ago-- I never had a problem before. Some salad dressings contain raw egg yolks. If I eat those I get bad stomach cramps (it almost fells like the cramps you get when laughing). I doubt it's the veggies unless maybe you are allergic to a certain pesticide or something (I also broke out in hives one day from the pesticide from an orange, Had to go to the emergency room and get a shot in the butt- OWW!)? Try buying your veggies from a different store or get the organic versions and see if that helps. If you eliminate or replace things one by one from your salads each day you hopefully should find it.