can my employer discharge me even when I provided a Doctor excuss?

I am a bargaining union employee at my company and my supervisor discharged me because my doctor excuse did not state that I was unable to work on that day, but it stated that I was excuse from work for that day.

My employer has accepted those forms of doctor excuses in the pass with no problem and which are in my records. I filed an grievance and after 90 days I was told by the Union that they withdraw because the written doctor excuse [pre-written form], used the term [as it always does], excused from work on such-n-such a day, and can return to work on such-n-such a day. But my supervisor did not feel I was sick and wanted the doctor to write "unable to work" before he would accept it. As I explained to the Union, my supervisor has accepted this form of medical excuse in the pass and with others employees, so why now would they reject the doctor's excuse being giving?

I believe my Supervisor felt I was not sick and refuse to accept the doctor excuse because the words used “excuse from work” does not mean “unable to work”. So the Union withdraws my case and I lost. What can I do now, when there is proof in my records as I told the Union Rep, that it has been a past practice of the company in accepting medical excuses written of this kind? It doesn't make prudent sense for the doctor's excuse to read something to please his suspicions that I was not ill at the time being excused.

Do I have a case against the Union for not pushing the issue of their past practices or with my employer? Maybe with both.

My Supervisor can not make the call as to his suspicious that I'm not sick. He is not a doctor.

However, a doctor would have authority to make that call and provide the pre-written form to be giving to your boss. My doctor used the term "Excused from work".

Please advise. Thank you.