Does Metformin mess with your period?

I was put on Metformin by my doctor for weight related issues regarding medication side effects. Still I've heard that this medicine helps make your period more regular. It seems though that while it makes it regular in the sense that it comes around the same time every 3 months or so when it does come it is light for a week, then medium-heavy for two weeks, and then light again the final week. And yes you read correctly. My period lasts an entire month. Is duration of the period due to the Metformin I'm taking or possibly other medications I'm taking (Invega, Luvox, Effexor)? 

And thanks for answering/reading!