Am I lesbian or bisexual?

I'm a 15 year old girl, and have a GF of 6 months. I'm what you'd consider 'tomboy', or 'butch' and she is fem. I feel more dominant and like I want to do everything to please her. It's a nice feeling, but it's a DIFFERENT feeling than I do when I think about being with a guy. When I think about it, I feel like a weight was lifted from my chest, more comfortable....yet, no attraction. Maybe some attraction to personality but nothing sexual. What is this? O.o