Is she bi.. should i tell her i am?

Well it could be a really bad reaction or a really good reaction. Bad~ she could feel that you betrayed her by not being honest about it and feel used because you guys swam topless and slept in the same bed (not saying that was your intention but there's a chance she will feel like that). Or she could be really accepting or even admit that she likes you too. You should be able to know kind of how she will react. I have several friends that I've made out with at the club and pretended to be bi with to get rid of guys and they would be okay if I was bi and attracted to them even tho they weren't. And I have friends that have joked about it but would be extremely freaked out. I would come clean about the bi thing first and see her reaction and if its good then tell her you think your attracted to her.