What Causes Inconsistencies In Periods?

I stopped drinking Diet Coke b/c I thought Iced Tea with Splenda or Equal was better for me. I have a lot of the artificial sweetener. I noticed for the past 3 period cycles that they have been a lot shorter (18-21 days instead of the regular 28). Could this be b/c of the iced tea? It is the only thing that directly correlates with the shorter cycles.