See if you can't find a local rescue that focuses on bringing in strays and fostering/rehoming them that's no kill. There's a lot out there so you probably have one in your local city unless it's a very very small town. They would be able to take her and take care of her and her kittens. I would also go back to where you found her and scope the area out to make sure she hasn't actually given birth yet and you left the kittens behind.