Food allergy
Food additive/chemical reaction
Gluten reaction (what you describe is similar to the reaction I have to gluten and carbs and when I worked happened at this time of day because I would eat it at this time of day as a "pick me up")
Dairy allergy - lactose or caesin in dairy
Adrenal Fatigue (happens a lot at this time of day with symptoms that are similar to what you describe)
Deficiency in the Vitamin B family of vitamins
Deficiency of other vitamins
Hypoglycemia (symptoms are similar to what you describe)