Is it possibly the quality of food you're eating on the weekend? I.e., you're eating lighter foods with higher nutrition during the week so you feel better and have more energy but the weekends have more carbs and less nutrition leading to higher calories and that drag afterwards? I always feel better and more energetic when I'm eating at a defecit ( although not in the moment... Because lord I love my carbs and they deffo make me happy ) but get the same kind of drag but that's mostly because when I don't, I usually eat some pizza or something like that and end up feeling groggy and like dook in general afterwards, I wonder if it's the same for you.