This was the first sign of Whiskey's hormone issues for us. It was followed by hair loss on her flanks, which was misdiagnosed as mites by one vet, so I went to a different vet who is a guinea pig specialist and she quickly diagnosed her with a hormone problem, possibly ovarian cysts.
She couldn't feel any cysts so in the first instance gave her a hormone jab to see how well this worked, within a couple of weeks her nips were back to normal and the hair had started to regrow - it was lovely, like baby hair! It's been nearly 3 months now and she's only just starting to get crusty nips again so she may well need another jab, my vet would rather give her 'HRT' every 3 months than operate on her as she is getting on a bit now bless her.
I am NOT saying this is what your lass has but it is something to bear in mind - it could just as easily be mastitis or something - but I would keep your eye on it.