Slight discomfort in upper back, such as a poking sensation. ultrasound said that the entrance to the left kidney was a bit inflamed, no kidney stone was able to be located? what can it be?

As a nephrologist, I have not read an US report as such and I am not sure that statement is included in the official report. US is not a reliable tool for detecting stone beyond the renal pelvis. I assume you had urinalysis done before the ultrasound. If blood is present (hematuria) CT for stone protocol should be done. I doubt pyelonephritis because you would be more symptomatic. Good luck.

The kidneys are not located in the upper back but the mid- flank area. Ultrasound does not diagnose inflammation. Go to a doc who would do a good physical exam and then order appropriate tests- if needed