Work Advice

My job is very physically demanding. My doctor has told me not to lift more than 25 lbs. I let my manager know and they said they wouldn't assign me light duty that I have to make a choice....either do my job and go against my doctor's advice or not work. I am at a lose here. I don't really need to work but I enjoy my job and enjoy having something to do. I also am in school for another degree so I have that stress too.