Girl i am seeing keeps leaving me with blue balls

I haven't been with a girl under 25 in at least 8 years, she is 24.
I'm not sure if this is how younger girls act or not.

Basically we met at a club, made out heavily and i have seen her twice since then, one time on Monday and last night.

First night i took her to a bar, had a few drinks then suggested we go to the beach, we made out in my car, she suggested the back seat and more kissing & touching, top off, breasts out, rubbing her over jeans etc, pretty much grinding over clothes giving my a penis rash - basically she was right into it for an hour, i tried for 15 minutes to get her pants off but she kept making excuses and brushing it off, then suddenly she starts directing towards 'needing to get home as it is late'.

Last night i invited her over to watch a movie, we had a drink and she initiated (asked to when she can kiss me) - so yeah a repeat of the car but with more maneuverability and heavier making out, she was obviously wanting it, tried to finger her but she kept doing the same thing as before and then a little while later saying she needs to go soon, but would come back for kino, kissing and teasing but this time intermittently rubbing my penis.
I didn't say anything as to not let on that I 'need' it but certainly wasn't as jovial, and let her leave again, this time not giving her the big goodbye pash etc.

What could be the deal here?