
Hi,
Not sure here, but could she be having a phantom pregnancy? Since it isnt actuall natural for a fully matured bitch to NOT be pregnant (i mean when dogs were wild, etc, is not GOOD for them to always be pregnant!!!!!!!!) then many bitches come into season, then their bodies fully expect them to be pregnant, so they have a phantom pregnancy. I would expect that living with a close relative who IS pregnant will also have helped confuse matters for her as well.
However, i could be wrong (hehehehe, can and am frequently its nothing new). See what everyone else has to say.
Em