Do you know if the baby is posterior or not?
What my midwife suggested was to find positions to encourage the baby to "pop out from under/on the ribs" to allieve that pressure. My muscle is actually being pinched between babes butt and my ribs
For me (and this posterior babe) that is tailors pose, or on hands/knees (aka cat/dog, or the position for pelvic tilts)...basically any of the suggestions to help discourage a posterior babe while sitting.
Last night I had NO pressure, then laid down and felt her swing around to a posterior positon and boom! rib pressure and pain *again*.
I hope you find relief. I'm interested to hear if the chiro can help!