I'm guessing diet...
Bullying is doubtful if your not seeing any aggression... you would know it was happening if it was.
Stop feeding for a few days(3-5). That generally cures it. Other methods are like chen mentioned(temp and salt)
Your best bet is to do this in a separate tank.
When shes back to normal, get them on a good pellet. Hikari cichlid Excel would be my first choice. Comes in sinking formula now, so try that. Good for your rays too. The shrimp is fine, but not exclusively.
Other than that, if she's wild caught, she could simply have a damaged swim bladder from the decompression when being brought up, and she will keep having issues.