4 days of intermittent low grade fever (99.7-100.8), spike ~4pm plateau overnight gradually recover to 99 in the morning; accompanied by headaches, tachycardia (~120bpm); bright red blood in urine, flank pain, cramps, no urinary pain. what can it be?

He should be promptly evaluated for a renal condition and anemia. A pediatric nephrology specialist is recommended. Screen for infection and stones. Please see the Emergency room at a Children's center, while notifying your Pediatrician. Keep your child well hydrated. Monitor temperature and breathing, watch for signs of shortness of breath.

You need a thoro clinical exam then appropriate tests for this cannot be diagnosed without that. Suggest you see a Urologist asap or go to the emergency room. Hope you feel better