Persistent right flank pain for weeks. lower back, kidney area under ribs, above belly button, gallbladder area. bad taste in mouth. urine test & culture neg. gallbladder u/s neg, bloodwork neg. what else could it be? kidney u/s next?

Kidney ultrasound can be done but CT of the abdomen with oral contrast +/- IV contrast (if kidney function is normal) is preferred provided no contraindication with radiation.

You need to be seen by a Urologist who will decide what further tests need to be done. Also your back (spine) should be evaluated

Your symptoms are not typical for kidney stones. I would recommend that you see your primar care doctor who can do a proper examination and recommend the appropriate tests. I do agree that a CT scan wit/without contrast and possibly with oral contrast is the likely next step