Sick betta

[Doyle145]

Hi guys, I have a tank with a betta sorority of 5 and unfortunately one of my girls has fallen ill and I would really appreciate if anyone could provide me with any information on how to help her. I've done lots of research but I’m just not sure. She’s has a swelling on her left side and her body seems to be curved. She’s struggling to swim with her head pointing upright and her body tilted and I had noticed a tiny swelling last night but over night it has grown bigger. I do regular water changes and I always test my water. This is the first sick fish I’ve had and I’m kinda panicking. I’ve removed her from the big tank and placed her in a separate container to quarantine her because all of the other fish are perfectly healthy and I’m not sure what’s happening. Her appetite is fine but she is inactive and only moving to rest in a new position. Thank you so much for your help.