Labor laws in Michigan?

I work for a nursing home in Michigan. I have been employed there for 2 1/2 months. I was in the hospital emergency room on my scheduled work day. Upon returning to work the next day with my Doctor's excuse, I was written up although the Doctor informed me to stay home from work and avoid strenuous activities. I have never missed work at this job before nor have I ever been written up for anything. Is It legal for my job to override a Doctor's order and write me up?