The weather is hot outside, was like 90 degrees. I started my car few times today to go to places and when I start the car, the window fogs up right in the middle of the car. The car has only 44k and it hasn't done that before today. Do you think it might be the heater core?

 

Probably just moisture from running the AC the last time you drove and if so it's not a problem. If it's the heater core it usually smells sweet, like maple syrup.