I hate when the boss does that. I used to work in retail and tjey always asked the nature of your illness. That is illegal her in U.S. We finally started being gross when they asked and gave very vile, descriptive answers. They stopped asking.
I have to ask permission to leave if i feel ill and have been told more than once to stay. After I puked on the classroom floor, the principal reluctantly let me leave.
Ted, you were not wrong, and along with your complaint letter, you should gather your own incident statements.
Hope you are feeling better. and good luck!