One of the signs are her teats. I'm sure if she WAS pregnant, the vet would've said something (unless he's an idiot). Almost all strays have intestinal parasites, that's why the vet gave you worming medicine, and that's probably why her belly is so bloated. Check her teats. If they start becoming engorged, chances are, she's with kittens.

Thank you for taking this poor little girl in.