She asked me if I "feel like a girl", commented that she thinks I'll use "she/they pronouns" in college, said she keeps subconsciously using "they/them pronouns" for me, told me she thinks I'll "figure things out soon", etc.

And this is exactly how social contagion happens. There's a peer pressure element to confirming to the group, and apparently being lesbian isn't acceptable whereas being trans is considered superior.

You've already told her you're a female lesbian, and if she keeps making these incredibly invalidating and homophobic remarks as if trying to undermine your own sense of identity, you have every right to tell her to fuck right off.

people using "they/them" for me without asking

What the pronoun crowd don't realize is that they can't police other people's use of pronouns. None of us here look at a man who identifies as trans and uses "she/her" in our heads to think of them. The only time we'd use their "preferred pronouns" is in a social setting where we can't safely rock the boat.