I've just read something that explains the use of "I'm good" in place of I'm fine. The sentence, written by an American, was this: "You groomed him good too". She was speaking of an animal that looked good in a photograph; it had been groomed to look good. So the American usage is explained by the fact that they use "good" where we would use "well" or "fine".
Interestingly, good has a Germanic origin and fine has a Latin origin. That's interesting because the swear words that are regarded as the most "offensive" tend to be the ones with a Germanic origin. Their Latin-derived equivalents are often not even regarded as swear words. e.g. fuck/have sexual intercourse with.