Definitely get her in to see a vet. Only they can tell you if she's pregnant and IF she is, that pregnancy needs to be checked because if the male dog was a larger breed, it could kill this little girl.
Was she scanned for a micro chip yet? If not, please please try that. Thank you for taking care of her!!
ETA: I just saw your reply on the chip issue. Sorry for repeating the question. I hope you can find her Mommy if her Mommy wants to be found. And, again, you are an angel for taking care of her..thank you!