You need to take a gravity reading and find out if it dropped down to your expected finishing gravity. If you fermented at too warm of a temperature (temperature of the wort, not the temperature of the beer), the yeast will tear though the sugars and finish really fast. Unfortunately, fermenting at too warm of a temperature has some undesired side effects, such as:
Creating fusel alcohols (causes headaches and a solventy, hot alcohol flavor and aroma).
Creates off flavors (esters) that are probably not desirable in the finished beer.
Sometimes the yeast will stall out, and not finish fully fermenting.
I'm not saying that a high temperature fermentation is what happened, but it is the first thing to look at.
What is high temperature?
That depends on what yeast you used. The yeast package or an online check of the yeast will give you the optimum suggested fermentation temperature range for that yeast. I usually try to ferment towards the lower end of the particular yeasts temperature range.