Thank you for answering.
I don't have anything to hide, I just want to be prepared for the interview. I know it's easy for a european to get F1 visa and I'm not going to work there, anyway, during my PhD because I don't have time.
The thing is that I don't know yet if I will look for a (legal) job in Italy or there after my PhD and I don't think that it's so unusual for who goes to study in the U.S.. But they want you to demonstrate that you will come back in your country after graduation, so having a girlfriend from there puts me in a bad position about it.
What I wanted to know is if they asked you what you did during your trips in the states and how deep they go.
My problem is that I have been there for Christmas too and I can't just say that I was there for a normal vacation.