Do I need a specific reason to be depressed?

I was messaging a friend at work and we were talking about work drama and then I said, out of the blue, I probably need a long sick leave. She asked why, I gathered all the courage and answered that I wasn't doing well...

She asked: why, what happened? Is there anything wrong going on in your life?

Obviously I can't tell her about my bipolar II so I didn't answer yet.

But, seriously... does something wrong have to be happening right now for me to feel like I can't handle life anymore?
How am I supposed to answer her?! I kinda regret talking cause now I gotta say something.

Ps: work is detrimental to my MH, but it doesn't feel enough to justify how I am.