If you feel like bringing up food that you have already eaten, then that could be anxiety. It's easy for people to say: "Go on a diet, and do some exercise". In this case, I do genuinely feel like you are trying. Eating is normal, it is when you binge eat bad foods that it becomes a problem. If you are eating a few bites of, lets say, nuts. That's completely fine, otherwise if you didn't eat anything at all that is even worse. Allow yourself flexibility in your eating habits. Having a snack here or there is fine, given that it isn't something that will undermine the efforts of exercise or the healthy eating. If you physically get nauseated, then maybe your body isn't used to eating the certain foods you are eating, or are they foods you have eaten in the past? Either way, I'm no doctor, heck I don't even do Biology as my A-Level. But, honestly, if you get nauseous, drink water, go outside, think of something else. If you keep thinking about it, it won't go away.