Hey Tiff

it could be from your baby pushing up into your ribcage or diapragm and causing referred pain to the back I sympathise, as I experienced this with both of my pregnancies.

I had 2 sources of comfort...reflexolgy and yoga

you could try this:
Adhomukha Virasana (Downward facing hero pose)
A position in which the forehead is resting reduces stress, calms the nervous system, and softens the mind. Helps to relieve nausea and fatigue.
• Sit back on your heels, big toes together, and knees slightly wider than your hips
• Extending out through your back, attempting to keep your buttocks sitting on your heels, stretch the arms forward and move the torso towards the floor ( as pregnancy advances use a chair to fold the arms, to create more space for your growing belly)
• Keep the arms shoulder width, spread the fingers, flattening the palms
• Rest the forehead to the floor (or forearms if using a chair)

you can also do this with the arms extending up a wall, close enough so you can rest your forehead to the wall

this really helps to open up the ribcage and alleviate pressure, plus encourage the bub to move
hope you get some relief soon

xx yogababy