Google food diary and you will get many examples of how to do it. The basic ones are just putting down the food you eat for each meal and snacks, and the amount of liquid you drink, and what type. You can get more fancy by noting weight of food, and how you feel before/during eating (e.g. stressed, relaxed), and time of eating/drinking. Leave room to make notes about feeling bad at some point during the day. Remember that it may take hours to get a side effect from a food. You should start to see patterns. If you start now you should have something to talk about with your doctor in three weeks.