The HIDA scan (short for Hepatobiliary Imino-diacetic acid scan) is a specialized test to examine the filling and function of the gall bladder. It is usually reserved for situations in which the gall bladder ultrasound
produced inconclusive results. It is also used to determine if the gall bladder is able to respond to natural stimuli and function normally. It sounds like your doctor
started off with the right test (the abdominal - gall bladder ultrasound). In most cases, if someone presents with abdominal pain
that worsens with food, and gall stones are found on the ultrasound, no additional testing is needed. This is why I agree that you should question the exact reason for this test, since it is not done routinely. It is possible that your doctor thought the ultrasound was inconclusive and needed the HIDA scan to be sure that you do in fact have gall stones. It is also possible that your doctor is concerned that your gall bladder is not functioning correctly.
Although the HIDA scan is quite safe, it would be pertinent for you to discuss with your doctor why the scan was ordered. This way you can be confident that you are not getting excessive testing and that some sort of benefit will come from dedicating your time towards the test. I hope this answer helps.