Well... it certainly could be a kidney stone! I would say that what you Dr. is doing would probably be my best advice except I would be for a CT scan instead of Ultra Sound. They aren't as conclusive as the CT's with dye. That way it could check out that whole area, not just kidneys.
They found a cyst and a fibrous tumor on my female organs when they did a CT scan for a kidney stone. I had no idea!
God bless you! I hope I helped.