You also did a lot more exercise than you were used to, so your body was shocked. But I agree with the rest, you need to eat correct for what you want to achieve, however there were plenty of calories in the McDonalds meals (which besides the amount of saturated fats, aren't actually that bad).
I'd suggest you either weren't hydrated enough, or you simply pushed yourself too hard without enough fuel inside you. It won't take long for your body to adapt.