I've been on Sprintec since January 2009. I find it to be a very good pill. Have not have had any mishaps or problems with it at all. I've been on it for over a year and still have about 5 day periods. Of course, my periods aren't nearly as bad as they were before I started birth control. After about two days of an almost regular period, it seems to lighten up alot on the last three days. It doesn't bother me all that much.
But since you've only been on it for a few months, I would say your body is just trying to adjust. In my personal experience, it has always taken a least a year for my periods to become lighter.