What are the BN's tankmates, is it possible that it's being out competed for food?
If it did appear to be feeding properly before this then it's possibly an internal bacterial infection, in a way it's similar to bloat but it causes the fish to lose weight rather than the bloating.
Something like Myxazin or eSHa 2000 which are broad spectrum bacteria treatments may be useful, try rubbing some veg with garlic, which is an appetite stimulant, to encourage it to feed, lets hope you have caught it in time.
Is she a solitary BN or do you have others, because if there are others in the tank then you need to seperate the affected fish to another tank to stop the risk of it spreading.