Hi and welcome to YT
!
I know you're scared and worried, but everything may be just fine...the problem is that it may take some time to come to that point, unfortunately. We've seen SO many young puppies here at YT who will have this or that odd elevated blood level with no real explanation...only to have it self resolve with time.
If it makes you feel better, maybe split the difference and have the BUN rechecked in 3 mths...?
As you know, these weird anomalies happen in humans too...elevated liver enzymes, weird kidney levels etc...and sometimes there too, there isn't a solid explanation.
Hang in there, I know the waiting and wondering is really tough! Your wee one sounds like a tough little nugget though
!