It's not that often that someone knows right away "who" they are, and this includes BDSM as well I think. It's about discovering, so saying that you are unsure isn't a bad thing with your girlfriend. You two discussing things that you might like, that she might like, and see what you two are willing to explore together can probably help figuring things out.

I know that talking about things helped me a lot with Keltar, to figure out even further more what I liked, what I was okay to try even if it didn't appeal to me right away, and who I was. It's a process that can take time.