gaining weight doesnt cause PCOS... PCOS begins at puberty. Its inherited unless some chemical factor or steriod was used, then it can be as a result of that. But the weight gain is generally resulting in insulin resistance that results from PCOS.
You should discuss a fasting insulin and then insulin tolerance test with your dr. This will help determine if you are insulin resistant. If you are, metformin will help curb this, which will help you to loose weight, maintain normal hormones, get your periods regular, mature viable eggs, etc. Good luck!
Its definately a factor of both, it can go either way. One thing I do know, having PCOS myself, is that it makes it really hard to drop weight. A lot of gynos just shove PCOS under the rug and tell you to come back when you want to conceive, but never tackle the problems that are also associated with PCOS, the hormonal imbalance, wieght gain, increase in hair growth, etc etc... Sandra Cabot has written some great books on Syndrome X and PCOS diets and how to lose weight when suffering the exxtra hurdles of hormonal issues.
Find a gyno or endocrinologist that specialises in PCOS.
Tough question, because the answer is both. Hormone therapy is usually the best way to deal with PCOS, so if you haven't yet, I would make an appointment to see my gyno.