Pseudopregnancy
Sleeping Dog, by sweet mustache
The effects of pseudopregnancy vary considerably among bitches - some show no outward signs at all, but others may show physical signs such as increased appetite, weight gain, a distended abdomen, or enlarged mammary glands…even lactation in extreme cases.
Some dogs may also exhibit behavioral effects such as being moody or withdrawn. One common behavioral effect is to display maternal or nesting behaviors, where a dog will nest and treat toys and other animals as if they were her puppies.
Again, there is much variation as to whether a dog displays the effects of pseudopregnancy.