Betta Fish: Which medicine should I use ?

My betta became suddenly ill yesterday. She was her usual energetic and hungry self in the morning when I went to work and when I came home she had lost some color, fins are clamped, was lethargic and refused to eat. She has now been laying at the bottom of the tank and only comes up for air. A couple of hours ago I also noticed she has what appears to be long stringy poop (or maybe a parasite?) The water parameters seem to be fine (amonia 0, nitrate 0, nitrite 0-5) and the tank temp is 80.2. I haven't added any new plants or decor in a couple weeks. I added a photo of what she looked like a couple days prior, the water color looks different because of the catappa leaves I added.

I have kanaplex coming tomorrow morning and I have API General Care right now. I'm trying to decide which one to use as my online searching has told me to not use both at the same time. I know one if for bacterial infection and the other for internal parasites but I dont know what is actual wrong with my fish. Has anyone's betta had similar symptoms and if so did you have success with either treatment ?