You probably stimulated your Vagus Nerve (google it--it's not 'Vegas') with the cold drink.
In some people, this nerve can be stimulated by the darndest things, and the result is that the heart rate slows, your blood pressure drops, and you faint. There's some discussion right here:
http://www.medhelp.org/posts/Heart-Disease/Vagal-Nerve-Help-Syncope-with-cold-drink/show/597636
You probably should report it to your doc, but in general, the condition is not dangerous--except for this: when you suddenly faint, you can hit your head on the way down and hurt yourself badly that way.
The treatment for vasovagal fainting from cold drinks (you can google that, too) is to AVOID cold drinks, and certainly not take large gulps of them.