Clown with mouth open and breathing hard

[Newsmyrna80]

First let me say that I've had my tank for 13 years and a mated pair of clowns for 5. Nothing has been added to my tank in years and I am a rigorous QT fanatic. That being said about a week ago my female stopped eating and she's usually a pig. Then she wouldn't close her mouth and she seems like she's struggling when lying in the duncans that they have claimed as home. He keeps trying to prop her up (very sad). She also is breathing fairly rapidly. I thought maybe she had lockjaw so I caught her and by manipulating the plastic strainer I was able to see she could physically close her mouth. Which leads me to a bacterial infection maybe?
Has anyone had this problem? I have a full medicine cabinet but I'm reluctant to do anything in fear of over stressing her. Any advice would be appreciated! Thank you!