What do you tell people if you're undiagnosed?

I've recently accepting having CFS (been an issue for almost 4 years now).

I am not officially diagnosed, but it's pretty obvious due to my symptoms and severity. I'm not ready to officially pursue the diagnosis immediately but want to slowly work towards it. I know it's a difficult process.

The question - is it legitimate to tell people I have ME/CFS without the official diagnosis? I feel like I shouldn't be telling anyone that until it's officially diagnosed, but it would help to explain to certain people what's going on.