I call and refer to my girlfriend as mommy.

Hi, Im a university student currently aged 19 and my girlfriend whom I have been with for 1 and a half years is 27. She is Mature down to earth,very caring,supportive, and very fun to be with. I enjoy my time with her and have alot in common with her despite the age difference. We also grew up with similar situations, eg abusive and hard to please parents. Even though I said we have alot in common we are also opposites in terms of I am a Masochist whilst she is a Sadist so you can obviously guess that she is the more dominant one in the relationship in terms of personality and financial status. She can be very dominant and aggressive and I find that really attractive. She does however remind me alot of my mother in some ways however I never really received much love from her but my girlfriend fills that gap for me. She is also drawn to me because of my personality and she has mentioned multiple times that I look alot like her father yet she calls me "baby". Overtime I began calling her mommy whilst we have sex and it came out by accident but the thing is she liked it and she said that she loves it when I call her that and since then every time we were to have sex she would always refer herself and tell me to call her mommy. To make things clear I do not want to have sex with my actual mother; I just see my girlfriend as a mother figure and someone I really love. Is this normal? can someone explain this more to me if they can?

Thank you.