I don't think there is much advice anyone can give you except your vet. Hopefully, you will be able to get in tomorrow.
As for parent's weight, that can give you an 'idea' but no guarantee. My girl is 11.5 lbs and healthy. Both her parents were under 7 lbs. She may not reach 5 lbs. You must also keep in mind that weight is not the only thing to consider when breeding.
If you are interested in breeding, maybe you can find a mentor breeder in your area to start learning from now.
Good luck at the vet tomorrow, hopefully she is not pregnant. Keep us posted.