Translation:I was fine before you opened your mouth.
It's normal to blame things on the young, but the adverbial sense of "good" is close to a thousand years old, but it hasn't been considered standard for the past 400 years or so. It continues in dialect and colloquial English. The OED has lots of delightful cites from 19 & 20c America. For instance, this sense, from 1946 K. Tennant, "We're doing pretty good".
Except, again, "I'm bad" or "I'm good" are supposed to refer to personal qualities, not to transient feelings. It's not semantics. Understanding the difference between the idiomatic "I'm good" and the grammatically correct "I'm well," is pretty crucial to learning how to speak and write English properly.
I almost always agree with nerevarine, but in this case I think I don't. English is a Germanic language, and "Ich bin gut" is perfectly grammatical.
A little under a thousand years ago, we started importing Latin-based words at breakneck speed. Many people wanted to import Latin-based grammatical rules, too, including the idea that "good" is always an adjective and never an adverb. But English speakers through the ages have insisted on saying "I'm good", "I feel good", etc.
So I'm not convinced that the supposed grammatical rule that you can't or shouldn't use "good" as an adverb is correct. I suspect it's no more wrong than splitting your infinitive.
Why does this require the subjunctive imperfect? Opening one's mouth doesn't seem like a hypothetical action here. Not trying to be obstinate, just trying to really nail down in my mind when to use it and when to not use it. I thought I had everything sorted out until this sentence.
I have to tell you that this is a common expression. It is aggressive and rude, but used nonetheless. One would often say, I was OK until you opened your mouth. Never I was good until you opened your mouth. This is often said when you are having a good day and someone ruins it by saying something stupid or inappropriate.