It could be that your body is telling you it needs electrolytes...that it's dehydrated....You need to have electrolytes put back in and not sports drinks either....
Try eating very lite after exercising. Have a few crackers, let that settle then have a few more. Good luck! :)
Is this sickness only when you are exercising? I do know if people push their body's beyond their fitness level then we get nauseated. I don't know if this is what is going on or their could be something else.
That is all I know. Good Luck
Jen