I'm

I'm not being funny

Used as a softening preface to a statement that might be taken as offensive or malicious.

I'm not telling you

Used to express that someone doesn't need to know something, typically because it is a secret (sometimes used as a way to attempt to hide someone's wrongdoing); may also be used when someone tries to ask for answers while doing an assignment or examination, or private or confidential information, including passwords.

I'm not the one

A warning to an individual who has greatly offended one's dignity or has acted or spoken insolently.