There is this girl, weve been a couple in the past, like 4 years ago but only lasted for 3 months.

After that I didnt see her for almost 3 years.

Now we go out sometimes. I know she likes me, and thats why I want to make her my "guinea pig". Im trying to understand why she likes me and how my actions and use all this information for myself during gaming other girls.

Last night we went to this small mountain who is like pretty dark on nights. We were on the top of the mountain , on top of a big water tank. After a while, I fucked her and after some min having sex she told me:

"...im not sure about this."
"I am" I replied and continued with my thing. She laughed and didnt put more resist to it.

I also could feel her heart beating so fucking fast before we had sex. I only put my head over her belly like just resting and her heart was like a race car.

What reactions, body language, things I should focus to get an Idea about why she likes me?

I've done this in the past based on similar reasons and trust me it's not worth it.

Here's the thing: People are not all the same. Honing in on a single individual and then applying what works with her to others only, rarely works because you need to hit that small percentage of people that likes the exact same things. If she's a very particular type you can learn to identify people like her and then apply your knowledge, but it would be more beneficiary for you to be testing your stuff on a multitude of women and then focusing on what seems to work with a wide variety of different types.

I have a variety of stuff that works well with almost anyone and then I have more specialized stuff that works well with types. When i meet someone i start wide and then narrow in on what sort of person they are.

_________________
No matter how much cats fight, there always seem to be plenty of kittens. ~Abraham Lincoln

Because i can.

whatever reason she gives you for liking you will have very little value or useful truth to it.