Quite simply, if someone questions your food choices, you can reply that you are choosing the foods you like and the foods that make you feel good. This may involve eating a donut (or two) one day and completely avoiding sweets another day. You may be trying to figure out the foods that don’t make you nauseous or that keep your blood sugar stable or that don’t give you hives. But you don’t need to go into that if you don’t want to. And you certainly don’t need to make excuses. You don’t need to say that you were “good” all day and deserve a treat, or that you just came from the gym and “earned” your meal.
You do not need to explain or defend your food choices to anyone. It’s okay to choose a food just because you felt like eating it.