I would bet it was from sodium in the chicken salad. It all depends on what your pre-op diet has been consisting of but when you don't know for sure what an item has in it sometimes the nutritional information can be horrifying. I know, for instance, I work in a hospital and we carry breakfast type sandwiches or cheeseburgers that you can throw in the microwave for patients for snacks. You would think, sure, there are probably some calories in here and some fat but not too high considering it's some cheap sandwich. I looked at the back of one of the packages the other day and there was damn near 900 calories in the cheeseburger. And this is nothing gormet by any means.
Also, when you are so strict with eating your body can do funny things, especially as you try to lose weight. I would just get back on your plan and try to relax. Weigh yourself in the morning after having gone to the bathroom. I think most of us post-oppers do that anyways to make sure we really have an understanding of where we are.
Good luck, you will be fine!
