I have a girlfriend that is 20 and we had sex for the first time on November 2nd and she found out two weeks later she was pregnant, so she goes to the doctors on jan the 24th and they said she was conceived October 25th and she is 15w along which would have been the 11th of October, and my mom is a 25 year nurse and she said it might not be my child and said from the conceived date she is already 2w pregnant which would be around Oct 11th. So what do you think if its my child or not?

That is a question only a DNA test can answer.

I doubt it is yours, there is almost no way she would show pregnant two weeks after the sex. Even 4 weeks after sex it does not always show up.

So my guess it is not your child. I would ask for a DNA test when child is born