There are a few things that we would need to know before we can answer your question.
If your tank is cycled, everything cannot be at 0 for your water parameters. A properly cycled tank should have readings of 0 ammonia, 0 nitrite and some level of nitrate (10-20ppm maximum). Recheck your readings and, when you do the nitrates, be sure to shake bottle #2 for at least 3 minutes.
Please give us your exact stocking (how many fish of each species is in there) and how long has it been stocked? Have you made any recent additions? If so, did you quarantine the new fish?
Has this particular fish been in there a period of time? If so, is this new behavior? Or, is she a new addition?