Calorie burn would also depend on age and speed.
I'd say somewhere around 900 ish as a rough guess, but it depends on a lot of other factors.
If you're feeling crap, its a combination of things.
What are you eating before this?
Whats your hydration like before, during and after?
Plus, bear in mind that you have nearly covered a half marathon. You gave your body two days rest then pumped another 3 miles into it. Poor nutrition and hydration between would leave you feeling anything but optimal.
Congratulations by the way, very few people I know ever go further than 8-10 miles.