How do I stop my adult, spayed female cat from running outside?

My cat is about 3 years old, spayed as a kitten, and has ALWAYS been an indoor only cat. Lately she has taken to running out the front door, usually when I am bringing my dog back in from a walk and my handscare toofull or I am too distracted to stop her. Obviously I don't want my little brat to be pooping all over the apartment complex, jumping on peoples cars and generally making a nuisance of herself outside. Not to mention being killed by poison, big dogs, hawk, cars, disease...
Why would she start this so suddenly, and how do I stop her?

J C2016-09-26T04:11:11Z

One thing that may work - put a squirt bottle in front of the door. If you can, tie the dog with his leash at the end of the hallway, porch, etc. Pick up the squirt bottle and slowly open the door - as soon as you see her peeking, let her have it! They also make things called "scat mats" that prevent a pet from going in an area, check those out. You can get them online.

Why is she doing this? Only she knows but I suspect there is a scent that is new to the hallway/doorstep that she wants to check out.

Uptown2016-09-25T23:00:32Z

shoot it in the head, its the easiest way