Check the ingredients on your yogurt. Most have some form of sweetener.
Dannon's Activia lists the ingredients: low fat milk, strawberry, fructose syrup, sugar, etc.
Since they split the sweeteners into two sources (fructose syrup and sugar), they didn't have to list the sweetener as the first ingredient (by weight).
Just know that you are eating a lot of high-glycemic sweetener in most cases. The ill effects of too much sugar are well documented.
Hope this helped. Dan