It may make you retain some water, but how "high" of a number are you talking about here?
Weight fluctuates on a day to day, hour to hour, basis. It's not static. You can "gain" 5-10lbs (or more) per day just by food and water intake, exercise, and hormonal changes.
The best time to weigh yourself is in the morning, naked, and after you've gone to the bathroom.
It wouldn't hurt to cut out the diet soda however, it does nothing good for you anyways. Try seltzer water with a splash of citrus juice instead.
I purchased a Soda Stream soda maker for my daughter. You fill a bottle with water and attach it to the machine to carbonate it, then you can add in any flavor that you want. I don't really drink soda but a few times I've added crushed raspberries and strawberries or lemon to the water and it tasted great, even without the syrup you're suppose to use with it.