Metaformin changing bleeding?

My doctor put me on Metaformin for weight loss, with the added benefit of it regulating my cycle. Since I went on it I've had fairly regular cycle lengths. I have PCOS and my cycles were 19 to 90 days--just all over the place. Now they're 28 to 33 days apart, but instead of bleeding heavy for 5 days and light for 2 (for a total of 7 days), I now spot 3 days, bleed medium for one day, and then spot for another 5 to 6 days (for a total of 9 or 10 days). I'm concerned something isn't right because of my bleeding pattern. Has anyone else had that experience or have any advice or information?