I'm going to talk about my housemate behind their back
What makes you want to talk behind your housemate's back?
They don't know that I am ill and I'm tired of pretending like I am okay.
I'm sorry you are feeling that way. Housemates should discuss their issues with each other. You can't expect them to know if you are keeping it a secret, you should cut them some slack.
No, I'm going to talk about them behind their back.
That is not going to help the situation. If you don't want them to know about your condition, then find someone else to talk to, but don't talk behind their back.