Why am I gaining weight on the Dukan Diet?

So I'm on the attack phase of the diet, and have decided to only do it for three days because I really only want to lose around ten pounds. I'm on my last day on the attack phase and I have managed to gain three pounds. Before I started I was 135, and suddenly I'm 138. I stayed strictly to the diet, so I'm confused as to why I don't see any weight loss. I workout as directed and drink plenty of water. And if this information is useful, I think I'm due to start my period soon. I have seen reduction in cellulite on my body, its all pretty much gone. Thanks for your input :)