I know the feeling. I had it the whole time I was banded... years. I still have it on occasion, actually, from diaphragmatic damage.
IF it's not gone in a couple of days, please check in with your doctor, OK? I was told it was "just gas" for months and it caused more and more damage. I'm not telling you this to frighten you, believe me. It's important to not leave it if it's more than just gas. Right now, it's most likely from the CO2 gas... stretch a little, if you can, walk, heating pad... pain medication doesn't seem to really help with the referred left shoulder pain for most people.
FEEL BETTER SOON. I hope it goes away quickly for you.