Ok, there are 3 possibilities that I can think of:
1) Ammonia: your tank is not cycled yet, so you could be seeing the effects of ammonia toxicity. Do you have a test kit (crucial during the cycling period)? If so, what is your ammonia level?
2) Illness: she may have been ill when you brought her home from the store.
3) Bloat/Swim Bladder Disorder: does her stomach look distended? I agree with not using freeze-dried food since it can cause bloat. Bloat, in turn, can put pressure on the swim bladder (the organ responsible for maintaining neutral buoyancy).
If you can provide a bit more info about your water parameters and her general appearance that would be helpful.