My daughter had  blood in her diaper once when she was about 3 months old. We also spent  numerous hours in the doctors office trying to get a urine sample. Every time she would tinkle it would all leak out around the bag and the doctor didn’t have a tube small enough to cath her. After all of this, we got the same explanation you were given. I also questioned this, but now, 2 months later, there has been no more blood and she is doing wonderful. Hope this helps.