If she is going through her first heat her nipples can enlarge to grow to adult size from puppy size this is normal
My Female Pitbull's Nipples Have Started To Hang And Have Gotten Larger,is She Pregnant?
My pit is 15months I noticed her nipples are getting larger and they fill hard could she be pregnant
Yes, it can be mastitis, or it can be caused by a "false pregnancy". Usually mastitis isn't just the nipples being swollen but the whole mammary and is usually accompanied by other symptoms. In a false pregnancy usually only the nipples protrude. It could also be an irritation of sorts but that is not as likely.
Yes.it can be an infected lymph node or a tumor
Possibly.
Is her stomach swollen? If so, its likely, especially if she has started nesting and is gaining weight, dogs can be pregnant as early as 6 months I think, check a website
That's possible. But it's also possible that she's merely gaining weight.