I believe it was Freud that said that there is an "inate bisexuality" in everyone. I think that many people believe that being straight is the norm and that one diverges from that point. I believe though that everyone starts right in the middle between homosexual and heterosexual. We have no preconceptions at birth, and as infants we have as much personal attraction same or opposite sex playmates. It is through cultural, societal, educational learning that we change into our "norm" (whatever that is...)
I personally have gone through many phases. I believe I was mostly straight as an adolescent, in my early teens I thought I might be trans (I always thought I would have done better in life as a girl), in my late teens I was a sexually active homosexual, I've been bisexual (75% hetero/25% homo) for the first 15 years of my marriage and now I'm still bisexual but 20% hetero and 80% homosexual.
That's a lot of phases and I'm still only in my mid-40's. I've still got lots of oppotunities to change in the second half of my life.
So my answer to your question is yes, people can go through phases of sexuality. And because of that, there is no reason to label yourself and feel bad about yourself. Let yourself feel what you feel, it's natural. Denying your feelings is denying your life.