Yes, it is very reasonable that something might change.

I like the tea analogy, just imagine that instead of sex you were just sitting for a cup of tea. If you offered tea, she might be excited and happy about it and consent to it. Once she starts drinking the tea maybe something happens, maybe it hurts her stomach? (Totally possible and happens in sex) maybe she changed her mind by the time the tea was made, maybe it didn’t taste good, or a million other things. The important part is that she can change her mind, and if that happens, sure you might be disappointed that you worked on preparation and got excited about it and maybe have some extra dishes to wash, but would you be mad at her for it? You shouldn’t be, she didn’t do anything wrong, she didn’t attack you personally, she has no obligation to drink the tea you made, and she has every right to change her mind about drinking it.

Just like tea, you shouldn’t force anyone or coerce them into drinking tea. And if they decide to stop, you definitely shouldn’t force them to finish it.

Don’t be offended, rejection is not a personal attack, just be polite and respectful and accept her choices.