Originally posted by: Elias
Unspayed female dogs have a dramatically increased risk in mammary cancer - which can be particularly nasty. Mammary tumors will likely be very hard to the touch, but I don't believe a tumor is the only possible hard lump that could be found in that location. I would seriously consider bringing her into the vet, as they can do an aspirate or biopsy to see what really is the problem.
IMO, chances are slim that she's begun lactating or something like that for no reason.
Edit: I don't know enough about false pregnancies to comment, but I guess that could be a possiblility as well.
I had a dog with a tumor, it was small and hard as a rock, this dog has just swollen breast glands that are very soft.