Proteinuria and Hematuria

I have a 5 year old son.  About 5 months ago he felt mild pain in his right flank after he urinated.  The next time he urinated, he had gross hematuria.  I won't go into the chronological order or exact details, but 3 months after this first epsidoe the same thing, flank pain after urinating, then gross hematuria in his next urination.  Then one week ago, the same thing.  So three times we've witnessed this.  In between these three episodes of gross hematuria, we have had a renal ultrasound, CT scan, urine tests and blood tests.  No masses, no abnormalities of the kidneys nor kidney stones where found in the ultrasound or CT scan.  Protein was found in the urine sample we captured that had the gross hematuria, but the next time he urinated, his urine was normal with no traces of protein or hematuria.  So the doctors are leaning towards kidney stones because the flank pain, hematuria and proteinuria are gone after one or two urinations.  He has no other symptoms or outward problems.  Our Pediatrician is "perplexed" and the Urologist made the assumption that it is kidney stones (after the 2nd time it occurred).  We have an appointment with a Nephrologist in 8 weeks (they don't think there is a reason to get him in sooner).
My question is, what could this be with these conflicting results?  Could it simply be just kidney stones that are small enough to pass quickly and without too much pain?  Can they form and pass that quickly?  Should we be looking at something more serious?

I appreciate any response.  Thank you.