Yeah, it's definitely possible. There are many factors that play into sexuality, especially environment related. Also, if you look at things like pornography, you hear time and time again of people's sexuality fluctuating based on what they're watching.
I also agree with the personality aspect. I consider myself an ambivert personally because I do need social interaction as much as I need my alone time, and every personality quiz I take usually scores me around 50/50 for introvert/extrovert qualities. There have, however, been times in my life where I've been extremely extroverted, and other times where I've been extremely introverted. That depends on my state of mind and circumstances at the time.
So yeah, both things can fluctuate.
