It's definitely worth speaking to your GP; have you had your post-natal check yet?
Although it's totally normal for your urine frequency to reduce after having a baby (due to the fact that the bladder once again has enough room to store a decent amount of urine!), only going once a day does seem very low.
You could have a mild urinary tract infection, but low urinary output can also be a sign of something more serious. Get it checked out, and hopefully you'll be back to normal soon!