Do you think it is normal that I don't want to be a father anymore?
Depends, why don't you want to be a father anymore?
Because I'm in the field of Biology, and my girlfriend's in the field of Physics. I have to study in order to meet my future obligations, and so I would only have time to take care of myself.
It's good to focus on your education and career before you have children.  I am sure that your girlfriend will understand, and if you want to get married and raise children together, you will both be better off doing so in the future.
I didn't want to be a father at this point in my life.
Well if you don't already have a child, then putting that off until you feel ready is smart. At some point in your life you might just change your mind.