There was a thread a few months back about Crystal light slurrpees purchased at 7-11 and although it was surgar free the carb count was way higher than you would expect. I can't remember the exact count.

but I did find a thread that quoted the nutritional info on diet pepsi slurpee and it said 2g sugar and 2 grams carbohydrate per 8ounce serving so I am unsure if that means 2 total carbs or 4 per 8ounces but That would mean that it has either 5.5 or 11 carbs in the 22 ounce slurppe you had so maybe thats why you went out of ketosis maybe just a few more carbs than you usually have?