@Kilo, I guess we didn't take into account age.
I'm now 24, and although I'm still pretty flexible, I don't think any of my friends can 'just' bend over and pick something up off the floor without a moment's discomfort/hesitation/pause. Whatever, it's not as easy as when they were 15
@Mirromn, I'm underweight and have struggled with energy issues through my life, so I can recognise it

Your blood sugar level was WAY down, and your body was struggling to replace your energy that day. Don't forget that a day of manual labour carrying heavy objects up and downstairs is WAAAAAY more taxing than a day at the office (or w/e), and you'll need to adjust your food intake accordingly. Fruit is a good food for giving you a healthy quick burst of energy, but ideally either bigger meals, or snacks inbetween are good on days like that IMHO.
@Kilo (again), I'm seriously underweight as well, to the point where on most body fat testers, I don't register as having body fat. You've just gotta be more careful, and recognise that doing the exercise will deplete your energy, therefore you need to take in more energy before you exercise, and refuel afterwards. Do that and eat plenty of proteins to help you rebuild muscle, and you shouldn't have a problem.
Routes played: Emi (10/10), Shizune (6/10), Lilly (9/10), Hanako (9/10), Rin (7.5/10)
Emi > Hanako = Lilly > Rin > Shizune
"As long as you're still alive, you can keep going." ~ Emi
100 pushup challenge
MADE HER PROUD - Final exhaustion test: 101