My screen cracked with out me dropping it do I have to pay for a new one???

Theoretically there are situations where Apple can and should replace the glass because it's considered defective.

However, the evaluation is usually over what the crack looks like and if there's other damage to the phone. Defects generally appear as single or minimal cracks with no missing pieces between the cracks, and with no signs of impact at the case near where the crack starts (usually at an edge).

If it's a lot of "spider web" cracks then it's not likely to be covered under warranty. I guess thermal stress could do something like that. However, that kind of shock is going to look like it's been hit with something even if the case is perfect.

I would make an appointment with the Genius Bar, or call Apple Support to see if it may be covered....however....from what you described, I doubt it. Still, I would check with Apple.