I had a similar experience.
Weighing daily and experimenting...I discovered that drinking diet sodas either kept my weight the same or even put a little weight on me.
Drinking water and reducing calories takes the weight off. If I eat the same number of calories, but drink diet soda...I either stay the same or add a little weight.
I drink mostly water, a little coffe and a little tea with one packet of Truvia. This seems to have helped. No more diet soda at all.