Hi.
I think it's just your body recovering from the exercise Focus.
When I used to go to the gym, which was many years ago now, ( which I still miss) when I'd get home, I'd be sooo tired, sometimes I'd nap, but because my kids were younger back then, mostly I couldn't go to sleep, so after a rest, I'd carry on...had too.
Even tho you say your only doing a steady workout, it's still exercise, and it's your body telling you to relax and recover...I think the more you do, as you go on, your body will get more used to it, but saying that, I've known people sleep for a couple hours after their workout.
The feeling of coldness is I'm assuming, that it's your core temperature cooling you down.
Which is good isn't it? You don't want to over heat...just make sure you drink plenty water as you go..xx