If she's brand new pregnant the vet couldn't have felt it. Later on, you can even see the kittens move inside. If her belly is bulging it can come from the food (BTW cats that have been outside on their own sometimes tend to overeating as they had never enough food -so be careful. She should get enough food but not overweight). If the whole cat is very slim and just the belly bulges it could be worms.
Good luck with her. I am sure she is very happy she found you (although she may not know at the moment)