What you describe (specially the "mental fog" part) sounds like what happened to me right before being diagnosed with a gluten allergy. Most days I wasn't able to think straight/work &c, and I felt into a sleepy state.
If paired with some other symptoms like bone pain, insomnia &c, I'd recommend trying out living on no wheat, lactose &c. for some days and note if the mental fog goes away. If it does => doctor. If it doesn't => doctor anyway, since it seems you do have some allergy related condition.
If paired with some other symptoms like bone pain, insomnia &c, I'd recommend trying out living on no wheat, lactose &c. for some days and note if the mental fog goes away. If it does => doctor. If it doesn't => doctor anyway, since it seems you do have some allergy related condition.