At 6mo old could get her pregnant. She would have to be in heat though. Being in heat could also make her nipples change. My missy went into heat at 1yr old, her body changed and she even gained weight and acted like she was pregnant. It turns out she had a false pregnancy, she sure fooled me!
I think the only way to tell is to watch her and see if she has any changes in her body. The vet should be able to tell at 4 weeks I think? Hopefully a breeder will see this and be of more help, you might want to post the question in the breeder section to be sure they see it
