That pregnancy was a month ago? There wasn't a buck with her when she gave birth, right? If there was, check her over and look for the new nest and litter.
If no buck was around it doesn't have anything to do with that litter. Take a very close look, is she scratched, bitten, or is the the blood coming from a nostril? Is fur missing? Check her all over.
Are there any rabbits around, apart from the kits? Could she have hurt herself trying to get her nose to another rabbit through the wire, any possibility she got bitten that way (I considered naming my buck scarface, he stuck his nose too often through the hutch doors of my Fury).
Does she act differently, does she react to treats normally? Is she more skittish than normal? Sometimes, when rabbits get spooked they can hurt themself running into things like fences.
Can you make close up pictures - with the available information this here is pure guesswork.
Anyway, if you aren't sure if it's serious, see a vet.