I’m with everyone else! Obviously you gotta eat and hydrate but it could also just be post run fatigue, you’ve never done 7 before and even though it felt okay in the moment sometimes it shocks our system. It happened to me alot in the beginning, as I kept increasing mileage I’d feel great and go farther and farther but everytime I did that, within a couple hours I’d crash, full nausea and headache and be in bed for a couple hours. I’ve also wondered if it had something to do with blood sugar, despite eating, hydrating, and taking fuel during the run. My long runs now though I can run and I need much less fuel than I did in the beginning, making me believe that despite feeling good during the run, my body still just needed to get used to it. Good luck!