If I work set hours part time each week can my office manager tell me they don't need me whenever she wants?

I recently went from full time status of 13 years to part time with agreement from the dr I work for. I am working two days a week now and signed a paper saying that I'm not entitled to sick vacation 401k and holiday pay. It said nothing about the dr going away and the office being open that I can't work. The office manager insists this is how it is. This isn't the first time she's causing problems for me and id like to know if this is legal.