Welcome to icliniq.com.
Recurrent fever with lymphadenopathy sometimes indicates chronic infection like tuberculosis. If ESR (erythrocyte sedimentation rate) is normal and the fever subsides, this may be a remote possibility. A mild fullness of pelvicalyceal system sometimes indicates mild obstruction lower in the urinary tract or incomplete evacuation of urine. I suggest you do a urine routine and culture test. If the consistency of the lymph node is soft, then it may be reactive due to the past infection. You already told that your child suffered from fever last month. So, these lymph nodes if less than 1 cm in size and soft in consistency and not attached to skin or underlying structure may indicate reactive etiology which is not harmful. But to be sure, you can do a Mantoux test. if that turns out to be positive, then there is a scope to worry.
The Probable causes:
Reactive lymphadenopathy, UTI.Investigations to be done:
Urine routine, culture, and sensitivity of urine, Mantoux test.Differential diagnosis: