Oh Trisha! I'm so sorry to hear about the loss of your new friend
Please do not blame yourself. It is very difficult to pinpoint exactly what happened, especially since you've had her for such a short period of time.
Budgies hide their illnesses REALLY well. She could have already been sick when you bought her and deteriorated quickly. Either way, it is NOT your fault.
I'm sure you gave her the best possible care and love than she would have ever received at Petco
R.I.P little one. Fly free over rainbow bridge.