Platy swimming similar to a snake! What's wrong?

Hi! My baby, 1/2" platy is swimming like a snake! For a while she was sitting on the bottom of the tank, but when I looked at her she got up, and started wiggling through the water. She's also gasping. The rest of the fish are fine, and the water is perfect, with 7 ph, 0 ammonia, 0 nitrite, and .1 nitrate, so what's wrong with her? After a little while she sinks back to the bottom. I tried looking it up, but nothing matched. Please help her, I've had her since a baby!!

Yah, sorry, I was reading the wrong sheet. It's at 5.