Jamie, I believe George and Joe are right on the money, with your arrows being "too stiff"....if that tiny bit of extra thickness made that much of a difference, then that is certainly the case.
Are you sure you draw 28"?
You also might have "been having a bad night"....might have started a little off, then you start thinking about not shooting good, or maybe your plate change and things go to pot quick...
I've messed around a lot with shimming out plate areas, and I've found that unless you make major, major thickness changes, it doesn't make much difference at all for me...but that's just me...