If the "owner" threw the cat out to fend for itself in the midst of winter, then why should she care if you did the right thing and got the cat spayed? She obviously doesn't care for the well-being of the cat. She isn't making sure the cat isn't going to get hit by cars, isn't making sure the cat is well-fed and always has fresh water available, doesn't care if the cat has intestinal parasites that could be migrating through her lungs or tapeworms that are sucking her dry(fleas CAUSE tapeworms)... To me, the cat is a stray now.

When I deal with strays, they get trapped, sent in to get fixed, get shots, dewormed, and out they go again unless they have a chance to get adopted(good temperment and everything).

If I were you, I would put the cat in a carrier, take it to the animal shelter and give them the cat. Make a small donation too. This way, the cat is at least warm, has plenty of food and water, will get fixed and vaccinated, and have a good chance of finding a loving home with someone who deserves her.

If you know she is possibly pregnant, why would you allow her to have kittens in 20-degree weather? She could easily die, the kittens could very easily die... Why allow it when you can PREVENT it?

At least take her to the shelter if you don't want to take her to the Spay/Neuter clinic. The shelter will get her all fixed up and find her a good home.