We said "I'm telling" where I grew up (USA) too.

There is familiar language that might be technically wrong, but it's how people express themselves. It's "I could've cared less", and "I got so bored of that, and "ain't", and "me and my friend went out to eat". Fine for communicating, as long as one is among a friendly bunch of people, but meself is teaching students, and try to help them know what is correct. I have a feeling students appreciate that.