Funny you should say that... while I've never noticed the ingredients of my yoghurts, and I only have one a day, I did start last week having a squirt of lemon juice with some cold water first thing (I never use lemon juice in cooking, as Dukan says you can, so I thought it'd be ok...).
Do you think it might be that? Interesting!
I really think I had too many carbs in the way of veg/salad at the weekend though, cos I "gained" 800g of *water* at the wkend, and have dropped 300g per day since, but today that still leaves me 200g up!