Day 16 Feeling Sluggish

I'm on day 16 of an extended fast. I was hoping to go until I feel hunger again (30-40 days). I am feeling very lethargic today however and I can't figure out why. I haven't over exerted myself today and I didn't yesterday either. Two days ago I felt great and did a 9 mile hike to The Wave. Can y'all help me out?