Probably because it depletes your personal resource level below the minimum required to stay awake. Especially if your protein level is low. You may need to eat more proteins afterwards. I found peanut butter, ham, and beef worked well for me, but you may have to choose something more in line with your tastes.
may be because you repeat the same same exercise times and again? lol
A wild stab in the dark....I would say your diet is not balanced with the amount of exercise you are doing.
And what do you eat after your run?
And how long are you running for?
I would agree. You should feel energized after your work out!