staffing policy changes

At our hospital we have a money crunch and a staffing crunch like everyone else. So far we have great pt-nurse ratios and no mandatory OT. I really respect the powers that be for keeping that standard, but recently my boss took me aside and asked that I pick up more extra shifts. I already pick up about a shift a week, but she was referring to more shifts on a particular unit. Since I already work that unit I prefer doing my extras elsewhere. She said it was a "job expectation" that I do extras on my home unit. I explained that I already fufill my obligations per my position as far as hours per week, and that extras were my perogative where, when, etc. Well we didn't exactly come to a meeting of the minds :angryfire

Now on the upcoming staff meeting we have "staffing" on the agenda. Based on the changes that have happened to others in the hospital I am betting that we get mandatory extra shifts to cover the summer vacations. I pulled my job description and it says nothing about working beyond my hired hours (of course). My boss's views differ from mine in that I think extras are a favor to the hospital, she says "not necessarily"

Could someone in management enlighten me as far as where she might be coming from? I have honestly listened and I don't get how an hourly worker could have a job expectation to work beyond her hired hours. I understand that during those hours I am hers totally-no objection to that. I just want to ensure some kind of limits to what I am required to do.