No problem! The thing you wrote is actually called Henderson-Hasselbalch equation. It fails in multiple cases (as you have to make a few assumptions during derivation), such as when the ratio of acid and base is drastically skewed, or the solution is either too concentrated or diluted. The H-H equation is still very useful for approximating the pH of some systems. Also, pH is dependent on temperature (think thermodynamics!), for example pH of water at 0 degrees C is something around 7.5 (please check the actual value, this is from my memory), but this doesn't mean that water gets alkaline when you lower the temperature. The story of pH and measuring it is much more complicated, and maybe it deserves a post on its own!
RE: Acid-Base Balance in the Human Body: Mechanisms of Regulation