I had surgery a couple months back. But it still gave me problems so the doctor wanted to do the injections to see if that would work.. I have a doctor excuse when I can return back to work. I got told if I don't return to work today I fired... can they do this...

A doctor's note gets you out of school, not work. Unless you belong to a union, or the company has specific written policy about sick time and LTD, or in some cases a gov't job, then the company can fire you. Some laws about this vary by state.