My best current guess, then, is carb withdrawal. If you did not experience it previously be grateful; you’re a little older now and that matters. Do you have any health issues? Without knowing how much and what specific foods you’re eating, your IF and eating window might be an issue but I’ll let those with more experience and knowledge discuss that with you. They will want to know how much and what specific foods you’re eating, so be prepared to provide that info. I do IF almost every day (dependent on my work shifts) simply by including my sleep time as part of the fasting time. If I can do so, I schedule my last meal and first meal 12 hours apart. So I guess that’s a 12/12.
PS: If you edit the title of this post to include IF you will get the attention of people who might be able to offer advice about that and have knowledge/experience with possible side-effects like lack of energy and motivation.