This is a really deep contraction. It's pretty common all over America to hear this in speaking, but especially heard in the Southern parts of America (where I'm from). Here's the breakdown: I am going to run on home. I'm gonna run on home. I'm a' run on home. Sometimes you'll see it written as: I'ma Ima There's really no spelling rule (that I know of) because it's extremely informal and improper English. You may also hear someone say, "I'm a gonna VERB" or "I'm gon VERB" which is just more bad English.