I was recently diagnosed with PCOS, and my Dr. and I have just start discussing Metformin, and I have just started working out again (I suffer from severe chronic daily migraines), but I know what you're going through as I have been there before.
There are a couple things to consider in this situation (aka: plateau). You may need to step up the intensity of you work out and/or you may need to eat more to accommodate your workout. I would often plateau and then realize that I had increased my activity without increasing calories, and when I would eat a bit more I started loosing weight again. Sometimes, though, you body just gets used to your level of activity and you have to mix it up, add in more intensity or intervals in order to break a plateau.
Take a look at your diet too, make sure you're getting proper nutrition; it's easy to go over do avoiding fats and carbs, but we need those too. I may be new to the idea of PCOS, but I've had insulin issues since I was a kid. I find the key is not over doing carbs and eating carbs in the form of whole grains along with fiber and protein.