N.K.
I suggest going to your endocrinologist. Most PCPs will end up referring you to one anyway. I don't know how things are in your city, but finding an endocrinologist that also treats PCOS is rare here and I had a heck of a time finding one. It is important to get blood work to see if some of your hormone levels are off. Hormone levels can change from one month to the next, especially when you tamper with your diet or your weight. If you were prescribed Metformin, it was for good reason and you should not have quit taking it without consulting with your doctor. The Metformin will help control your insulin levels, which in turn controls the way your body processes sugars from food, and weight.
Perhaps you need to go back to the Metformin and modify your diet. Modification of lifestyle (exercise and diet) could help you enough to not need the Metformin anymore, or to reduce your prescribed dosage. If Metformin and exercising helped you lose weight, why not continue with that? According to Google "Hormonal IUDs also list weight gain as a possible side effect. However, according to the Mirena website, fewer than 5 percent of women using it experience weight gain." I don't know if you could be in that 5%, but does not seem that likely.
Not exercising, eating anything you want, and not taking your medication WILL make you gain weight though.
MilitaryMom 6. has the right idea. You need to consume small portions of lean proteins and vegetables. Simply cutting down on portion sizes will make a big difference. I had no idea how large portions were until my endocrinologist explained it to me. Now I understand why there's such an obesity epidemic -- restaurant portions are HUGE! They also sneak things in like sugars and fats into the sauces and condiments, so you're consuming food that isn't exactly healthy, and large portions of it. Stay away from starches and stick to a low glycemic index diet, which is best for those with PCOS and diabetes. Again, your endocrinologist can provide you with a brochure as to what types of foods these are. Do some walking, eat at a decent time, and get a good amount of sleep as well, I'm sure you'll start seeing results!