Betta swimming in circles/loops; not eating

[Malachi]

My fish is a female Betta named Beru; I've had her for about 3 weeks.
Last Sunday (5 days ago), she started floating around at the top of the tank - but would eat food whenever it was dropped into the tank. Today (just a few minutes ago), I noticed that she wasn't swimming straight; she was swimming in circles and twisting around, and hasn't eaten food for a day or so.

I have some plant bulbs (labeled "Betta plants" at PetSmart) in the tank that she has been gnawing on, and now they're sprouting white tendrils and are covered in some slime. I washed the slime off of the plants when I changed the water Monday.

Currently, she's lying upside down at the bottom of the tank and her gills are moving a lot (sort of like she's hyperventilating).
What should I do?

[Malachi]

Beru stopped moving entirely a short while ago...
I'd still like to know what might have been wrong so that I don't make the same mistake again.

[dramaqueen]

Hello and welcome to the forum. Could you please give us some more info? What size tank or bowl is she in? Is the water heated and filtered? What do you feed her and how often? If she's in a tank, is it cycled?This info could help us figure out what is wrong with your betta.