I can't figure out what is wrong with my betta

[Audstar24]

My female betta fish has been in this tank for (I did the math) six months and five days. Since getting her it has been wonderful and she has looked so healthy and her colours really developed. It is a 20 gallon sorority tank with five females and I have no issues. In the past two weeks to a month something is happening with her fin and lower body. I have not noticed any changes in behavior with any of the fish in my tank and she acts as she always has. Her lower body and the outer part of the "infected" area is black and the inside of the part on her fin is a blue/white colour. The inner part on her fin is like a clump. I was waiting to see if I could find something or if it would heal on its own but i'm not sure now. I have three fish tanks and am getting a fourth later this weekend and have never had something like this before. Attached are images of her now and when I first got her. Please help!