It's probably the baby pushing on ur organs then. I've had to start leaning back slightly when I sit bc mine will press on my bladder and up into my ribs at the same time. If sitting a different way doesn't alleviate it, it could b heartburn. If using regular remedies for that doesn't work, try asking ur doc what to do so u can atleast find some sort of comfort.
Could b some sort of heartburn or it could b bc the baby is pushing ur organs up depending how far along u r.