Human Resource Question?

Last week I had to call out of work due to pneumonia. I turned in a doctor's note, called in advance...everything I was supposed to do. Well, yesterday was my first day back and I was written up for calling out. I thought an employee couldn't be punished for a documented illness? It upset me quite a bit, but before I go calling my HR department about it I am trying to gather some information.

Was that appropriate of my manager to right me up? What's the point of sick time and doctor's note if you're still going to be punished? I would appreciate insight. Also, I am reviewing my work handbook to try and understand the company's policy too.

Thanks.