Mass seen on ultrasound but not on the follow up CT scan?

I posted in this group about a month ago after a mass was found on my right kidney during an ultrasound.

My follow up CT scan was completed this week and the results are in today. It said there is no mass seen. Was the mass on the ultrasound an artifact? Or what in the world is going on.. I've been extra stressed this month waiting on this CT and apparently for no reason if there wasnt a mass to begin with 😭.