My guess would be it's most likely from your male trying to mount her and ripping the fur out as he does this. That's where they grab hold of the fur with their teeth to mate with the doe. The only way to stop it is to keep your doe and buck separate. I don't know how familiar you are with breeding, but if the male is still in with her, she's most likely already pregnant again. Which means the babies will need to be weaned early at 4 weeks when the new kits arrive. Usually it's best to not keep a breeding pair together for these reasons, as well as if he is posing any risk to the new litter.
If you're certain it's not the buck, other possibilities are a heavy molt spot if she's molting, or possibly mites. Though usually with fur mites there won't just suddenly be a bald patch, but the fur starts to thin more and more as the dandruff worsens.