Irregular after a Year of Regular Cycles with PCOS.

I've been regular thanks to Metformin (1700mg/day) for a year after I was diagnosed with PCOS. I've had 29-31 day cycles for the last year and now I'm late 10 whole days. I'm on CD 40. What the heck?! I thought once I was regular on metformin I wouldn't suddenly go irregular again. Anybody have an experience like this? We are trying for our first child.