I live in the southeast US. I first started hearing "flustrated" by local speakers a few years ago. It was apparent that the speakers were not using this for comic effect; the speakers were quite angry at the time.
I don't think that it's a combination of flustered and frustrated...just a variant pronounciation of "frustrated". If you look at the examples used by the other contributors on this site, I think that they illustrate the use of this term as "frustrated".
My boss keeps giving me contradictory directions of what he wants. It really gets me flustrated.