Yeah, your AC can get worse than 10, but it's REALLY sad if it does. Theoretically, if you were to advance an Ooze to Colossal size, it would in fact have 0 AC. But unless you're playing as an Ooze, don't worry about that. Why do you ask this question, anyway? I hope it's for sheer curiosity, because if you're actually playing a character with a Dex penalty and no armor or shield (the only way you could conceivably sink below 10 naturally), then you'd better have a great Con score, or you might want to rethink your strategy. :\