Never seen them on the back...but many young Rotties have white chest or back leg markings; or black smudgy markings on the front paws that often disappear with time. Sometimes it takes a year or so, when the coat's blown out (shedded) a couple of times.
I doubt it's anything to worry about! Rottweilers do get impetigo (like Michael Jackson!) but that usually shows up when they're adults, I think. And it's quite harmless, just cosmetic.
Heck, you're not planning to show her so it doesn't matter!
I have three Rotties, not one of them could enter a show ring (most wouldn't be suitable actually.) Just love her up & enjoy her "chrome."