How did you acclimate her to the new tank water when you got her? And when you did the half gallon water change? Was the temp the same as in her cup, and when you replaced it? Conditioner?
If you didn't take her out and acclimate her to the new water, and if you didn't acclimate to both temp and chemistry when you first placed her in, then it could easily (and sounds like) she is in a little shock and stressed.
What is the temp of the water? She needs a heater.. just use a thermometer to keep an eye on the temp.
She is stressed from the new move, so she will need a place to hide in to feel safe.
Plants, a cave.. something. She went from a small tiny cup to a (for her) big place with no where to hide, no where to go.. she is probably really stressed from feeling over exposed.
So it can definitely be a mix of how you acclimated her, and that she has inadequate hiding places to feel secure.