Depending on why your doctor ordered the test, an abdominal ultrasound, such as a gallbladder ultrasound, can be used to look for abnormalities anywhere in the abdomen. Most likely the doctor ordered the ultrasound because he suspected gallstones.
Some other reasons your doctor may have ordered your abdominal ultrasound:
Unexplained weight loss.
Your doctor felt a lump or mass.
Abnormal laboratory test result.
Common diseases the radiologist/sonologist might discover:
Abdominal aortic aneurysm (ballooning of aorta).
Dilated bile ducts.
Enlarged pancreas or pancreatitis.
Tumor or mass in liver.
Enlarged spleen or liver.
As to why you change positions and hold your breath, it allows the Sonographer to obtain the best quality pictures.
Btw, I had one and yes that is routine. I didn't think they would find anything but the results did show I had stones and some "sludge" in my GB. It was a surprise to me but my doctor figured it all along when I described my symptoms.