Cat Is Super Active

There's a stray cat in my are which I've been feeding and petting since Summer. When she approached me for the first time, she was pregnat. About one and a half month after that, she gave birth somewhere in the outdoors.I don't know if her kittens live or no, she's young, about 1,5 years old. Anyways, lately she's been way more active, she used to only play with moving stuff, and relax but now she climbs trees, she jumps on roofs and she even climbs my metal fence, she does not jump on it, she literally stands up, grabs from the fence and gets on top of it, I am not kidding. She also used to avoid other cats, now she hits them lightly(she does not hurt them), she sneaks behind them and chases them. And, I've seen a little kitten(young) around the streets, which is white, my cat's color is brown and white. Why did my cat suddently became super active? Can the little kitten have something to do with it? My cat is the only cat that was pregnat in the neighbourhood, including ALL the other stray cats.

It sounds as though she is feeling good and since she is quite young, she has lots of energy.  Some cats love to climb and be in high places.

The kindest thing you can do for her is to get her spayed.  Also if you find any of her kittens, they too will need to be spayed or neutered.  It is very difficult for a cat to continually get pregnant.  They can have litter after litter.  This also contributes to the massive stray/feral cat population. 

Thank you for caring for you, yet please consider getting her spayed ASAP or she will just wind up pregnant again very soon.