Hello,
Firstly, one might have undiagnosed renal disease or functional loss on the unaffected side. It might be interesting to check differential function in this patient if/when the episode resolves; i.e. the affected (infected) kidney might be supplying the bulk of kidney function in this individual. As well, without more details in the case, some patients with pyelonephritis might have general systemic sepsis, or experience the effect of antibiotics among other reasons that could effect general renal function, even if one side is the source of infection.
Dr. Jordan Weinstein