Month old yellow belly turtle died

by rebecca
(staffordshire)

I brought 2 turtles in january and they are growing fine and always seem to be healthy and happy , eating and swimming around. Two weeks ago i brought another baby turtle and added her into the tank she always on top of the rock basking in the rays and would hardly ever go into the water or eat. I have woken up this morning to find her floating in the water dead, i dont understand why considering i have had the other 2 about 4 months and they are fine. Why would she have died was it something i did or in the tank or was it purely she was ill when brought her ?