Can I get fired for refusing to take a job offer?

I recently got hired in February 2011 for an "on call" registered nurse position at a skilled nursing facility. I've been working about 2-3 days a week there. The director of nursing recently offered me a full time graveyard shift position and I declined, but thanked her for considering me when offering the full time position. Next month's work schedule has been released and I noticed my name is not on the list. I was thinking my manager is mad that I didn't accept her offer and is cutting me out of the schedule. Can she terminate me for not taking her offer? When she hired me, I was hired as "on call" or per diem. I never wanted full time hours. Can she fire me because I refused her offer?