It's a regular in my kitchen. It's not a high risk food, it grows like cabbage above the earth. Contrast with sprouts which are grown & stored in a moist medium where bacteria can flourish.
You didn't purchase the prewashed, bagged type did you? Lots of people don't wash their bagged greens (salad greens, spinach). They really should be washed & picked through. Once greens are washed they start to deteriorate.
Might be something else. Some types of food borne illness can take days or weeks to manifest so it can be tough to trace the source.
(I've taken a food safety course in a culinary program in case you're wondering if I have a clue.)
Debra