Did you make it yourself or did you buy it at a store deli? I've gotten food poisoning from eating tuna salad from a deli on the same day I bought it. I suspect the deli made it using mayonnaise that had been sitting out.
Tuna salad I make myself will usually last two days in the refrigerator, but tastes more "off" the older it is. I've never gotten sick from it even at 3 days old, but it doesn't taste as fresh at 3 days.