I think your University's response has been inappropriate and if they are using the word "disability" then they SHOULD know exactly why.

You are entitled to privacy, there are at least a dozen vague and applicable terms to describe why you were gone and what you will need without specifically saying "bipolar". For example: 'Personal health reasons', 'stress-related study issues' etc.

I strongly advise you to find someone who is employed by the University to be your advocate. My college had a Disabilities Coordinator who came in with guns blazing whenever I needed her. It changed my life.

You are currently being discriminated against in a way that is highly unethical and possibly illegal. Your professors do NOT have a right to your private medical information.

You should not have to accommodate them, they should should be reasonably accommodating you.