Bad back/rib pain

I am going on 31 weeks tomorrow. For the past 2 weeks or so, I have had
really bad back/rib pain. It's located right in the middle of my back and
stretches around to my side. It's primarily on the left side, but sometimes
the right side hurts as well. The pain is getting excruciating. It wakes
me up in the middle of the night. I've tried propping up pillows to sleep
in a sitting position and that didn't work. After laying on a heating pad
for awhile, it starts to hurt. If I'm sitting in a chair for too long, it
starts to hurt. If I'm standing too long, it starts to hurt.

The best way to describe this pain is a really painful sunburn on the
inside. It just burns incredibly bad.

I've started doing some stretching and it seems to help a bit, but isn't a
miracle cure.

The rest of my back feels fine. It's just this one area. Anyone else
experiencing pain like this in this area of the back? Thanks!