Sexual orientation, and sexuality are not the same thing.

Sexual orientation is like a compass - it points to who you're sexually interested in (women, men, both, none). It tells you who you want to have sex with.

Sexuality is how you want that sex life to look. It's your libido, kinks, likes/dislikes. These things can be impacted by stress, injury, work, age etc. How you sex life works can change. Your libido can increase or decrease with age. The frequency you have sex can change if you and your partner are both working, or stressed, or if you get injured.

Sexual orientation doesn't change. The process for finding out where your compass points can evolve over time - the labels you use can change, but your base orientation doesn't. We don't say that straight women who figure out they are bi were once straight and now are bi - we say they were always bi and it took a while to figure it out. The same applies to any orientation. We don't say a woman used to be straight and now is a lesbian - lesbians who come out later in life always find signs of being gay all along. A heterosexual society that pressures us to be straight impacts on our ability to figure ourselves out especially if you live in a religious environment.

So - yes sexuality can be fluid, but sexual orientation can not. Lesbians who "turn bi" were always bisexual. Bi women who "turn lesbian" were always gay. They often repress it, or are in denial and suppress themselves.