J.M.
Hi J.,
If it hurts still after one week, I'd go to the doctor.
I had back pain after the birth of my second child for months and months. Finally went to the doctor. It turns out my abdominal muscles didn't get back into shape. It's these muscles that hold us up, not our back muscles. After a couple of visits to a physical therapist and daily stretches, I felt great.
I liked your other comments too. It could be because you're bending over so much to change diapers and such. And the other comment - if it hurts when you move, you may have pulled something.
Good luck!!!