Oh boy, I would hunt down the nurse manager and review the contract with them. sounds like the hospital had a change in what shift they needed covered and just USED you.
I had this happen and my company DID NOT BACK ME. I did speak with nurse manager and her response was
" those are the hours I need you"
I finished and dumped the company.
Sorry to hear that. It is BREACH of CONTRACT, however.
Take care,
K