Wow, I think fasting on Diet Soda is the WORST possible thing you could do for yourself... you're doing nothing but circulating horrible chemicals through your body... Furthermore, there are some studies that show you will actually gain weight by eating...drinking...consuming things w/ aspartame, Splenda, etc --So that may be what you're experiencing.
Good luck on your fast thought... I do suggest you switch to juice, broth, veggie juice, and of course water for the remainder of your fast. I think your results will change drastically.