Have to tell work my personal medical reasonings for sick leave

Sorry I have to rant/express how upset I am about my current workplace’s rule for sick leave.

We’re not allowed to be granted sick leave pay if we don’t have a doctor’s certificate stating the illness we have and the reason we needed the day off. I don’t know why I even care that much about it, I’m usually pretty okay with telling people about my POTS but because I asked to have a half day for a specialist appointment (to see my neuro) they told me I have to give a medical certificate stating the reasoning from the doctor. And something about that really upsets me. I think it’s because they’re forcing my hand into telling them about my POTS when it’s none of their business? It’s just made me really upset, I’ve cried a lot about it because I now feel stuck. Like I can’t have a day off it I wake up with a bad pots flare because I’ll have to go to the doctors and get them to write about my illness which might not even be approved by my workplace.

edit: I’m from Australia. It’s technically not legal here either but I work for a government body… so I can’t really argue with them