Hi, I had sex with my girl without wearing condom. But never leaked inside mor have put it back inside after leaking. But she says she is pregnant. Please help me understand this.

It is possible to leak without any awareness of it. Pre-ejaculate (which you can google) can contain some sperm. You can also be mistaken about withdrawing before you started to ejaculate.
Here are all the possibilities:
She is pregnant and it's yours
She is pregnant and it isn't yours
She is mistaken about being pregnant
She is lying about being pregnant

How many days since that time you had unprotected sex?
What is her basis for the claim that she is pregnant?
You won't know whether she is really pregnant until you are with her at the doctor's, seeing the ultrasound done. You won't know if you are the father until the baby is born and you have a DNA test.

ANY time there is penile insertion there is the risk of pregnancy. You can'[t know whether you leaked or not. So pregnancy is possible.

You will have to wait until the child is born and have a DNA test to confirm paternity.

People who have unprotected sex are commonly called "parents". It is quite possible she is pregnant. Are you ready for being a father? If not, further precautions are necessary.

There is no form of birth control on the market that's 100%. Condoms are one of the least reliable forms, too much room for human error.

There are women on this site, and all over the world, that got pregnant using not one, not two, but 3 forms of birth control, some more than once.

So it's entirely possible for her to be pregnant and for you to be the father.

If you are questioning paternity you'll have to wait for the baby to be born, then have a DNA test done.