Hi Michele,
I hate to bring it up, but if the "panic attacks" have really just started out of nowhere, it isn't impossible that she could have experienced some kind of traumatic experience while you were not around that she hasn't told you about. It sounds like you have only left her with very trusted adults, and that's great, but it might be worth describing her behavior to a child therapist, just to get their take on the situation. Personally, my parents let me be babysat by people they didn't know very well at that age, and I was exposed to some harmful situations that I never told them about. As a result, like you, I have only let my children be watched by close relatives. Even though I consider them pretty trustworthy, my in-laws, were once watching my niece, and my mother-in-law was going in & out of the garage to do laundry & accidentally locked my 3 year old niece in the dark garage. Somehow, she didn't realize it for quite a while, and by the time she let her out, my niece was so traumatized, she still will not let anyone close a door to a room that she is in- whether it's the bedroom, the bathroom, or anywhere- she has to have the door open. So, I don't want to worry you unduly, but if my sons were acting scared out of the blue, I would definitely try to find out what could be causing the fear.
Hang in there,
C.