I had what we think was a gallstone attack about 12 hours after giving birth to DD. It was hell. I'd had rib pain on my right side since about 30wks but it migrated to shoulder/upper back/chest pain a couple of times. I went to hospital with it at about 36wks but they thought it was a muscular issue. Then the attack I had after labour was quite severe - the pain was unbearable (upper back/shoulder/chest again) and the only thing that gave some relief was lying on the floor. They kept putting me on the bed and I kept lying on the floor lol! I was on heart monitors and all kinds of pain killers until about 6 hours after the attack started they gave me a shot of morphine and I finally passed out. The pain was so bad I couldn't open my eyes and I now have a very patchy memory of what happened that night. It was awful.
11 months on and I finally got around to having a scan this week! No gallstones, so it looks like I passed a stone. That's the current theory. I only got it checked because we're about to TTC #2 and I wanted to avoid having another attack!
It's good that you're checking it now, if it's really bad and it is in fact gallstones, they might be able to do something about it now before you end up with an attack like what I had. Wouldn't wish it on my worst enemy.
I hope you feel better soon!