Well what does your fish look like? Is she bloated at all? Any scales sticking out?
Well that could be either parasite or bacterial in nature. I'd start by getting her in pure epsom salts with no additives - magnesium sulfate per ingredients list only. Predissolve 1 tsp per gallon into a cup and then add that slowly over the course of an hour. She can stay safely in this for weeks or months at a time so keep her in that during treatment.
To treat for bacteria use Kanaplex or Maracyn Plus. To treat parasites I would mix a feed using a source of metronidazole, focus, and garlic guard (pure garlic juice), vitachem or other liquid of choice even tank/dechlorinated water. Put a little of the liquid of choice in a small cup, along with one measure each of the metro and focus. Mix that up well and get as much of the meds to dissolve as you can. It won't be perfect. Put a good soaking pellet in there - a crappy wheat based one that swells a lot like Hikari, and let them soak 10-15min. Feed 2 twice a day for an absolute minimum of 2 weeks and at least one full week past when she's pooping normally and seems completely cleared. Most people just continue to feed 3 weeks to a month because relapse is high if you discontinue too soon. If you can't find pure metro you can use something with metro in it like API General Cure with metro and prazi. If you don't want to feed you could try using something like Jungle Parasite Clear in the water for a minimum of 3 weeks and one week past when she seems clear - this is a lot harsher so I'd try the feed.