I would wager the Warning letter you refused to sign is in your file.
if it were me, (due to threats of repercussions.) I would file a grievance for harassment being forced to come in, another for disciplinary action without a requested Steward present (you DID request one, and they refused?)
In the future, though, see the Dr the 1st day you are sick with a note when you should be able to return to work.
You have even stronger ground to stand on if this happens again.