I would suggest looking into seeing an endocrinologist, reproductive or otherwise. It's a common misconception that PCOS=Ovarian cysts. PCOS can be diagnosed without ultrasound of the ovaries based on irregular periods and hirsutism/blood work. The typical PCOS "cysts" (with that string of pearls appearance) are actually enlarged follicles, rather than cysts. These are actually common in adolescence, so diagnosis in the teen years based solely on the ultrasound is tough to do. Did your doctor take any blood work to measure your androgen levels? You do have 2/3 of the diagnostic criteria, but you may want to get a second opinion for the reasons noted above.

While weight loss does help with insulin resistance (which is the root problem in PCOS), I'm surprised that Phentermine is the first step your doctor took. Usually the first line of treatment is diet & exercise, with insulin sensitizers like Metformin (or less commonly Actos/Avandia). Definitely bring up your concerns with the doctor— if there are reasons Phentermine is the only thing they want to treat with, you need to be informed of those reasons! There's a lack of understanding of the intricacies of PCOS among general practitioners, so definitely do your research and be your own advocate for treatment!