There are a lot of comments saying that many women just don’t want to be seen as easy so they say no to put up a front, or that they may have changed their minds once things got hot and heavy.

While these are definitely plausible explanations, I think it should be pointed out here that it’s relatively common for sexually inexperienced, younger women to have trouble saying no, especially saying no more than once. I definitely did when I was younger. Namely, a girl might mention she doesn’t want to have sex during a heavy makeout session or whatever that might lead to sex, the guy agrees but continues making out with her and the girl goes along with it because why not (it feels good and hasn’t progressed further than she wants it to), and by the time it escalates the girl doesn’t know how to say “hey wait, I don’t actually want to have sex or do a/b/c” because she’s gone along with it up until that point. She might feel stupid at not stopping the encounter earlier, she might want to avoid what she feels is a super awkward situation, she might be worried you’ll get annoyed at her if she thinks she led you on. Whatever the case may be, she may go along with it and have sex anyway even though she would ideally prefer not. While this sounds amazingly silly in hindsight, girls are socialized to be non-confrontational and many women take time to find their voices.

This is definitely a grey zone when it comes to consent. My advice, OP, is to back off when a girl says she doesn’t want to have sex. Stop the sexual encounter at that point, and if she actually does want to have sex with you, she can resume it on her own terms.