I would speak with your Union Rep about the matter first.  Your union contract - which I am assuming here that there is one - should I hope have the specifics in it regarding the cancellation of a shift, etc.  If you challenge the contract issues ultimately a court decides what was reasonable under the circumstances (or a mediator or arbitrator is that is what the contract calls for). It may indeed limit you and your rights.  Maybe the procedures that are in place are unreasonable under the circumstances but one would really need to read the union contract in order to see what type of relief you are entitled to of any.  Good luck to you.