It could be a stone which passed - if it had lodged, you'd be in the hospital - they hurt! -, but it could also be a muscle strain, gas, or even anxiety.
I am not a doc, and my words aren't medical advice. I've done about five straight weeks of honcode/pro level reading on the subject, before & after having had my gallbladder removed. Pulled a total panic jag over it.
Do you fit at least 3 of the 5 f's: 40, female, fair, fat, and fertile? Does anyone in your immediate family have gallstones? Did the pain strike suddenly, after a meal? Any other symptoms? Ultrasound is not foolproof, but it's the gold standard for diagnosis. Call your doctor and share your concerns asap. Don't suffer!