Pregnancy with PCOS and being on Metformin?

I know, I know, another pregnancy question, sorry. I was diagnosed with PCOS in March 2008 and since then I have been taking Metformin and it has made my periods regular, every month like clock work. I started taking metformin because my husband and I have been trying for a baby and I had horribly irregular periods, like I wouldn't have one for 4 months and then had one that lasted a month. Like I said I have been on this medicine for a year almost and my last period started January 1, which means I am atleast 17 days late. I am going to tomorrow to buy a pregnancy test, but I am scared. I am scared that it will be no, but i can't think of any other reasons why I would be almost 3 weeks late. I have not been stressed or on any other meds, nothing.. Any advice. Thanks!