Hello,
Firstly, WELL DONE for managing to lose so much weight, and especially on Dianette, which, some studies have shown to be a pill that can trigger weight gain. You new dr. was right to be concerned about the length of time you had been on that pill, it carries a much higher risk of DVT than other pills. Yasmin is the usual choice after Dianette, because it is the most anti-androgenic and due to it being pretty new and using a progestogen that is an analogue of spironolactone (Spiro - used for excess hair and acne), it tends to be pretty effective. Yasmin also tends to cause less undersireable side effects such as bloating (the diuretic effect from drospirenone helps with cyclical water weight), acne and breakthrough bleeding.
On the other hand, while it may be great for some symptoms, it may cause you an increase in hunger. Studies on 2nd and 3rd generation progestogens have shown that 3rd gen progestogens have less effect on lipids - meaning they are unlikely to cause weight gain- in comparison to older pills that contain (PCOS-unfriendly) progestogens like levenorgestrel (Microgynon, Ovranette). Therefore, pills that contain desogestrel (Marvelon, Gedarel, Mercilon) and norgestimate (Cilest) shouldn't cause weight gain. Yasmin hasn't been studied properly for this, though it should follow the pattern.
Having said that, you may find that Yasmin doesn't suit you weight-wise. I would have said it could be oestrogen-related, but as you've been on Dianette and lost weight, that doesn't make sense. For all oral contraceptives, weight gain is a possible side effect. My own experience tells me that I put weight on without following a strict, low-calorie diet and exercising. I am rubbish at doing both right now, and so have gained weight. The pill that I lost the most weight on was Marvelon, though Cilest (which I am on currently) didn't inhibit my weight loss when I was on it a couple of years ago, and I know that if I tried really hard

I could do better!
Yasmin may well be causing the weight difficulties, though you won't find any proper studies on it due to the control that the drug company has on the media available - it's big business. If you're not feeling so good about being on Yasmin, I'd say try Cilest or Marvelon. Both are good for PCOS.
Hope this helps xx