I wonder if the stress on your back of carrying a baby is doing this to you. I suggest seeing a doctor and having some x-rays taken to start. Your GP is fine to begin with. If it's warranted, he can order further testing, perhaps an MRI, and refer you to a specialist.
Congratulations on your little one! Now please take care of yourself for your baby's sake. It's easy for us moms to put our own pains on the back burner as we focus on our kiRAB and take care of their neeRAB. Hang in there! Let us know what the doctor says.
Emily