It's likely you're thinking of one of these:
* "I'm" - Short for "I am" - Used in normal conversation.
* "Imma" - A slang abbreviation of "I'm going to" - Used in casual conversation.
* "IM" - Could be an abbreviation of "Instant Messaging" - Used in a more formal context.
If you can provide more context, I can help you figure out what you meant!