Not unless you have strong love feelings towards her and felt sexually attracted towards her.
Would I be considered a lesbian if I saw a really pretty girl and couldn't help but to keep looking at her, since she is really pretty?
I think of sexual orientation as a spectrum ranging from heterosexuality to bisexuality to homosexuality and everything in between those. [relating to people attracted to cis genders). What label you pick on the LGBTQPA+ spectrum (lesbian, gay, trans, questioning/queer, pansexual, and asexual) is just a label you feel best describes you.
Regardless of the label you assign yourself, you are the same person you have always been. The label just describes how you feel.
Admiring how pretty a girl is doesn't mean you're homosexual.
That depends on how you were looking at her. Like where your mind went at the time. It just sounds like you thought she was good looking lots of people think other people of the same gender have nice eyes, hair etc.