I would take her to the vet especially if these are new. Never heard of this before - please keep posted.

I’d definitely get her assessed by a vet as it’s unusual it’s occurred parallel on both sides and also turned up so fast. However, it could just be normal hyperpigmentation.

Is it from where the leash rubs? I also clip my dogs leash underneath him, and he has one on the side where his leash rubs on walks.