LED lamp increased power consumption after installation: Why?
With the limited information provided, this is a guess, and a suggestion for clarification, but...
Where did you get consumption of 2W per lightbulb from?  Those are simple LED's, no special drivers or current limiters included in the package. 

If you have them in a series string, you need to limit the MAXIMUM forward current for the entire string to 150mA (from the datasheet as absolute max If).  You can't do this just with a voltage source, and for a series string, I wouldn't attempt it with a single resistor either, use a current regulator.

If you have them in parallel, you can use a series resistor with each individual LED, based on your supplied voltage, to limit its current.

The wild guess/speculation part is here... I am thinking that without proper current control to the LED's, you are burning them out, and they are failing as short circuits causing more current to flow as the voltage drop across failed ones vanishe.