you aren't used to ketosis
What happens is that the first time you ketodiet, your brain can still only use 100% glucose for fuel, so the liver creates glucose from protein using a process called gluconeogenesis. After three weeks in ketosis, the brain can get up to 70-75% of it's fuel from ketones, which is when you really start feeling 'normal' again.
i would never cardio without ingesting some protein first or else your muscle gains will not be as good.

Yes, for the best results doing cardio on a empty stomach gives the best results. However when you have no fuel in your stomach your body will go into starvation mode eating whatever it can for fuel Ie) Muscle or fat.

So too prevent this eating something light like egg whites in the morning or whenever will be more benefical too cutting and keeping as much muscle mass as you can.

ok thanx BEAST !!!!! i will try it........

I do cardio on an empty stomach only because I'm in the gym at 5:30 am. I find I'm sluggish getting going but once heart rate gets up I gave no problems with energy.

All I know is swim pratice on an empty stomach SUCKS!