Is she in this tank on her own or with other fish? If she's on her own then as you say she likes to hide under the filter it's possible she got stuck and damaged her scales trying to get free - or are there any objects in the tank that she could have scraped against accidentally?
I would treat with Melafix if this is the case as that help them repair and prevents infection. Make sure you measure the dose correctly as too much can do more harm than good. In fact, I always under-dose on Melafix (well, not me personally as I'd rather have a cup of tea LOL) and even at half-doses it always seems enough to help the fish repair.
Regular partial water changes help too.
Regards, Athena