Well I'm not sure about what's wrong with her, but my sister's Double Tailed Betta died. He wasn't acting normal either and had hard time swimming so she made him a bed with half a styrofoam cup and some aquarium marbles to help weigh it down so he can sleep near the surface of tank and easier to breath. She wasn't able to cure him unfortunately, so he passed away.
I aadopted a betta last year from my classmate who didn't want to keep her betta, and he laid on his side a lot and had trouble swimming too because her fiancee put the tank on the windowsill and it fell over so the rocks and the little tank feel on top of him. So I took him in a nursed him back to helth, well how I did is frequent water change some tablespoons of aquarium salt and lowered the water level to 1/3 of the tank. Unfortunately I had to keep him in the little platic tank that was maybe 1/3 a gallon??? cuz I didn't have extra tank, and I had to use an incadescent lamp to heat the water. Well, he is doing great still. Can swim like a normal betta and has a voracious appetitle. I gave him to my sister so she can have a betta friend.
So you might have to take her out and quarentine maybe. Maybe make a little bed for her with half a styro cup so she can rest easy.
I aadopted a betta last year from my classmate who didn't want to keep her betta, and he laid on his side a lot and had trouble swimming too because her fiancee put the tank on the windowsill and it fell over so the rocks and the little tank feel on top of him. So I took him in a nursed him back to helth, well how I did is frequent water change some tablespoons of aquarium salt and lowered the water level to 1/3 of the tank. Unfortunately I had to keep him in the little platic tank that was maybe 1/3 a gallon??? cuz I didn't have extra tank, and I had to use an incadescent lamp to heat the water. Well, he is doing great still. Can swim like a normal betta and has a voracious appetitle. I gave him to my sister so she can have a betta friend.
So you might have to take her out and quarentine maybe. Maybe make a little bed for her with half a styro cup so she can rest easy.