There's every chance that it was excitement, as yacka said. You could also have just been at the end of the REM cycle, in which case your dream would dissolve anyway, and because you were already aware, you were aware of the awakening after the dream. You awaken after every dream you have, you just don't remember it usually because you are still half asleep, this was just you waking up and staying aware, as you were in the dream.

Don't let it get to you, it won't happen next time, just stay aware of your dream and stay aware of your surroundings within the dream and it shouldn't collapse