You didn't drink much water. Dark urine is highly concentrated. You probably need to drink a bit more.
Basically you are not getting enough of an intake of liquids (especially clear liquids) to flush out your kidneys. Not flushing out your kidneys well enough can lead to kidney stones.
Drink more water or tea (hot or iced.)
Too concentrated. Increase fluids.
If painful, renal calculi. So to say, Kidney Stones.
If no pain, you are on the way.
dehydration, kidney issues.