Your wife no longer has the control of her own time the way she once did. Perhaps she also misses the challenges and rewards of her work. Hobbies may have been put on hold for a while. Whether her pregnancy was comfortable or not, she may be mourning the special intimacy of feeling her daughter inside her. Many new moms describe feeling empty inside.
Pregnancy is a time of looking forward to an eagerly awaited moment.
Now that looking forward is gone as well. Also, pregnancy breaks down barriers in society. Complete strangers would beam at her, want to pat her tummy, and tell her she was glowing. They would leap up to give her a hand. Wearing maternity clothes when pregnancy is over just isn't fun, but usually nothing else is comfortable yet.