its more to do with the bulking agent in powdered sweetner - can make it a bit carby for such a small measure, and it adds up over the teaspoons.
Don't stress about it, you can't time travel and undo it.
I admit to having powdered sweetner if I'm out with a black coffee and the cafe or wherever doesn't have any tablets. Its a very rare occassion though.
P.S Buy 2 packs of tablet sweetner, and keep one in your bag, its handy if you are out etc,