Loose Job Due To Doctors Note?

I currently work at subway and i currently had to go to a doctors appointment for lower back pains . The doctor told me that she wants me to not work for a week and to stay off my feet until an appointment has been made for physical therapy. My doctor wrote a note and i made a copy and gave it to my boss. When she saw it she laughed at the letter like it was a joke and told me i may not have a job when i come back january 7. Is she allowed to fire me due to the doctors note? And is she allowed to cut most of my hours upon returning?