So I am trying to understand this phenomenon. Seen it before, now experiencing again. Make out with a girl at my place after a date out. She has been very sweet and kind of submissive up until this point. Ever since the makeout, she's been full of playful negs directed at me, and just generally acting not as 'impressed' with me.

WTF is this? Is it sexual tension building until the next date?

Some women view their sex appeal as the ultimate weapon in their arsenal. Maybe now that she knows you want it she thinks she can dog you. Think of it as a giant shit test. If you let her bitch you down you are going to lose this one. If she goes again at you then try saying "Well I guess I know who won't be getting more sugar from me" just flip it on her.

_________________
You can't forget about me, stupid. Everywhere I go ima have my own theme music.

Thanks for the insightful comment. This makes a lot of sense.

I should add that after the most recent date following the major makeout, she starts in with 'i don't normally do that' and other rationalizations.

So you encourage the rationalizations!
Kind of chill and safe way...
Say "I know, me either! But it just really felt okay in that situation. If we hang out again you can relax because there is probably like a 43% chance that I won't try something again, chuckle. But seriously, we are both adults and no harm no foul."

This will continue to let her rationalize and even console her to not think of herself as "being a slut"

My approach would be a little different on this one though...
I would say, "Whoah, I didn't mean to make you feel slutty. If anything I'm the slut! I mean you pretty much had your way with me...not that I regret it. Wow, I mean you really are a better kisser than I would have thought..."

This addresses the subtext of "I feel like a slut". It flips the situation around in a playful way and then ends in a playful sexual tone when you change threads. Not as safe probably but it's more my style.

_________________
You can't forget about me, stupid. Everywhere I go ima have my own theme music.