What do you think, my black mollie appears to be dying

I just did a water change, for my normal maintence, all parameters are perfect, while doing the water change i noticed that my female black mollie was swimming at the bottom, sometimes just laying there with her tail curled. She started to swim around like she was ok, and now she is hiding doing the same thing, and breathing heavy. I am going away tomorrow and dont want to have to worry about her dying and contaminating my tank. My other black mollie is doing just fine, except she is going after all the food that sinks to the bottom and chases the other fish away. Never seen this before

i went to go test parameters, and it was too late, she has died, found her at the bottom of the tank lying sideways. Nitrates are 10, Nitrites are 0 ammonia 0 ph 6.7, using api master test kit. thanks for the help though