Please help with angel fish death.

[Wrench]

I had a female swallowtail angel that I bought just over two weeks ago. She always ate from day one up through last night. I thought all was fine but I just found her in the rocks, dead. There are NO external signs of stress or disease. No lesions, sores, torn fins, growths or anything else out of the ordinary. Any thoughts as to what may have happened? All of the other fish in the tank are fine.