Like steve said, it's most likely that she got tired of the pellets.
It's like if we only ate poptarts everyday, even if we change the flavour, they're still poptarts.
In my experience, turtles tend to have "favourite" foods, which sounds silly, but variety is definitely key.
Sometimes, Yaji will only eat pellets or lettuce bits, and Yumi will only eat Shrimp, and then other days, Yumi will eat the pellets and Yaji gobbles up the shrimp fervently.
I just assume they know what they want/need at that given moment