- A large intake of refined carbohydrates
- You have an allergy to wheat or gluten.
- You have an allergy to yeast.
Refined carbohydrates are quickly converted by your body into sugar. This can give you a quick boost of energy, then leave you flat. This is similar to the effect of sugar, but with less severe effects . Your post-bread fatigue could be from this effect, if it turns out not to be the other two.
You can mitigate this effect by having less bread at a meal, and by having whole-grain bread rather than white bread. A local bakery is more likely to provide truly whole-grain bread. Large bakeries tend to use white flour with a few flour refining byproducts and call the sum of the parts “whole grain”.
Most bread is made mostly or entirely of wheat. A wheat or gluten allergy can have a number of symptoms, including fatigue. This may show up soon after consuming wheat, or it may take up to a day. If you eat bread every day, then you will likely feel tired all the time, although the effect could be even worse shortly after eating bread.