I would suggest not doing anything you are uncomfortable with. So if you don't feel comfortable wearing (or not wearing) certain clothes, then don't - you should look relaxed and natural.
The photos don't *have* to be sexy. The most beautiful single portraits I have seen have not been "sexy" ones. You can either get really creative with hair and makeup, or keep it simple.
The photographer should dictate the poses, usually, because it depends on the type of photos they need to add to their portfolio. Does she want some more sultry poses? Happy/carefree? Pensive?
My favourite ones which were done of me were when I was smiling and laughing with my son. My son wasn't actually in the photos, but the happiness on my face was very genuine; the smiles didn't look fake. Another favourite was one I did for a hair salon, where I had to look sultry. I wore a black corset and did my best sultry look into the camera...I felt a bit silly at the time but it turned out well! The ones I did the other day were for a woman who is opening up a jewellery/clothes store (I modelled the jewellery and clothes) and I mostly did 'pensive' looks. This is really hard to explain