I assume your not covered by a union contract at work, correct? If you're sick, DON'T go into work. Your boss sounds like a jerk and plans to punish you for being sick. Have your girlfriend take in the doctor's note and see what happens. While the doctor's note won't prevent them from firing you if they so choose, going to work when you have pneumonia is not a good idea either.
You are unfortunately experiencing one of the biggest problems in America. Almost all workers not covered by a union contract as considered "At Will" employees. As an at-will employee, you can be hired or fired for no reason or any reason at all, so long as the reason is not based on a federally protected status or right (e.g., race, creed, color, nationality, sex, age, etc) or for creating a sexually harrassing workplace. With the exception of adverse action based on one of those protected categories, your employer could in theory fire you because he doesn't like your hair color and you would have little or no recourse. You could of course collect unemployment unless the employer could prove you were discharged "for cause", that doesn't help much when you need every penny you get from your check. I am sorry I don't have better news. But there needs to be some changes in America if we're going to prevent situations like yours.
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