A girl which i met only couple of days ago (and had sex with her) just texts me to ask:

"want to come over tonight? " of course to fuck.

I don't feel like going.

What is going on with me? Is it too easy? And therefore not attractive?

Something wrong with me?

she isn't smoking hot, but nice enough. spanish girl, and good in bed. id say HB 7.5

It happens all the time bro, nothing is wrong with it. It's how male mental is designed ever since we were still living in the cave. Why do you think we develop physical body that is suitable for hunting and go out to hunt sabertooth and mammoth instead of settling for buffalo or deers?
When the hunt is too easy, you don't feel any challenge and you lose all the adrenaline. Same thing applies here. I usually lose all the interests if the girl sleeps with me on the first day. It might be boring, but do not burn the bridge, she will have some use later. She might be friend with some 10s or 9s, keep her around.

Happens to me all the time, man.

Nothing to worry about. Sometimes you're in the mood, sometimes you're not.