You mentioned having quinoa. That could be the problem ... perhaps it wasn't rinsed well enough? Quinoa has a natural resin coating it that protects the grain from critters. The resin can cause stomach problems if it wasn't rinsed off well.
Thanks! The quinoa salad was from the deli of a local health food store. I actually did figure out what the culprit was. Believe it or not, it was my vitamin tablets. I keep a log of everything that I put in my mouth, and I was able to backtrack my stomach aches to my supposedly "everything-free-vegan" vitamins. So, I am going to switch to a different brand that I have used in the past.